Semiconductor device and driving method thereof

ABSTRACT

To reduce adverse effect of variations in threshold voltage. A semiconductor device includes a transistor including a gate connected to one electrode of a capacitor and one terminal of a SW 1 , a source and a drain one of which is connected to one terminal of a SW 2  and one terminal of a SW 3  and the other of which is connected to the other terminal of the SW 1  and one terminal of a SW 4 ; a first wiring electrically connected to the other terminal of the SW 2 ; a second wiring electrically connected to the other terminal of the SW 4 ; a load including electrodes one of which is connected to one electrode of the capacitor and the other terminal of the SW 3 ; and a third wiring connected to the other electrode of the load.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices, display devices,light-emitting devices, methods for manufacturing these devices, andmethod for driving these devices. In particular, the present inventionrelates to a display device including a current-driving-typelight-emitting element which changes in luminance depending on current.The present invention relates to an electronic device including thedisplay device.

2. Description of the Related Art

In recent years, flat panel displays such as liquid crystal displays(LCDs) are becoming widespread. Researches on the display (ELD)including an organic EL element, which is not an LCD are activelycarried out (Patent Document 1). The organic EL is acurrent-driving-type light-emitting element changing in luminancedepending on current and also referred to as an electroluminescentelement, an organic light-emitting diode, an OLED, or the like. Forexample, methods for correcting variations in threshold voltage oftransistors have been examined (see Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2003-195810.

SUMMARY OF THE INVENTION

It is an object of one embodiment of the present invention to provide astructure with which adverse effect of variations in threshold voltageof transistors can be reduced. Alternatively, it is an object of oneembodiment of the present invention to provide a novel structure withwhich adverse effect of variations in mobility of transistors can bereduced. Alternatively, it is an object of one embodiment of the presentinvention to provide a novel structure with which adverse effect ofdeterioration of a transistor can be reduced. Alternatively, it is anobject of one embodiment of the present invention to provide a novelstructure with which adverse effect of deterioration of a displayelement can be reduced. Alternatively, it is an object of one embodimentof the present invention to provide a novel structure with which displayunevenness can be reduced. Alternatively, it is an object of oneembodiment of the present invention to provide a novel structure withwhich an image can be displayed with high display quality.Alternatively, it is an object of one embodiment of the presentinvention to provide a structure which can achieve a desired circuitwith a small number of transistors. Alternatively, it is an object ofone embodiment of the present invention to provide a structure which canachieve a desired circuit with a small number of wirings.

Note that the descriptions of these problems do not disturb theexistence of other problems. Note that in one embodiment of the presentinvention, there is no need to achieve all the objects. Other objectswill be apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a semiconductor device whichincludes a transistor including a gate electrically connected to one ofelectrodes of a capacitor and one of terminals of a first switch, asource and a drain one of which is electrically connected to one ofterminals of a second switch and one of terminals of a third switch andthe other of which is electrically connected to the other of theterminals of the first switch and one of terminals of a fourth switch; afirst wiring electrically connected to the other of the terminals of thesecond switch; a second wiring electrically connected to the other ofthe terminals of the fourth switch, a load including electrodes one ofwhich is electrically connected to the one of the electrodes of thecapacitor and the other of the terminals of the third switch; and athird wiring connected to the other of the electrodes of the load. Thefirst wiring is electrically connected to a circuit having a function ofsupplying a first potential and a second potential. The second wiring iselectrically connected to a circuit having a function of supplying athird potential. The third wiring is electrically connected to a circuithaving a function of supplying a fourth potential. The first potentialis lower than the fourth potential. The second potential is used forcontrolling the amount of current flowing between the second wiringsupplied with the third potential and the third wiring supplied with thefourth potential by the transistor.

One embodiment of the present invention is a semiconductor device whichincludes a transistor including a gate electrically connected to one ofelectrodes of a capacitor and one of terminals of a first switch, asource and a drain one of which is electrically connected to one ofterminals of a second switch and one of terminals of a third switch andthe other of which is electrically connected to the other of theterminals of the first switch and one of terminals of a fourth switch; afirst wiring electrically connected to the other of the terminals of thesecond switch; a second wiring electrically connected to the other ofthe terminals of the fourth switch, a load including electrodes one ofwhich is electrically connected to the one of the electrodes of thecapacitor and the other of the terminals of the third switch; and athird wiring connected to the other of the electrodes of the load. Thefirst wiring is electrically connected to a circuit having a function ofsupplying a first potential. The second wiring is electrically connectedto a circuit having a function of supplying a second potential and athird potential. The third wiring is electrically connected to a circuithaving a function of supplying a fourth potential. The second potentialis lower than the fourth potential. The first potential is used forcontrolling the amount of current flowing between the second wiringsupplied with the third potential and the third wiring supplied with thefourth potential by the transistor.

One embodiment of the present invention is a semiconductor device whichincludes a transistor including a gate electrically connected to one ofelectrodes of a capacitor and one of terminals of a first switch, asource and a drain one of which is electrically connected to one ofterminals of a second switch, one of terminals of a third switch, andone of terminals of a fourth switch and the other of which iselectrically connected to the other of the terminals of the first switchand one of terminals of a fifth switch; a first wiring electricallyconnected to the other of the terminals of the second switch; a secondwiring electrically connected to the other of the terminals of thefourth switch; a third wiring electrically connected to the other of theterminals of the fifth switch; a load including electrodes one of whichis electrically connected to the one of the electrodes of the capacitorand the other of the terminals of the third switch; and a fourth wiringconnected to the other of the electrodes of the load. The first wiringis electrically connected to a circuit having a function of supplying afirst potential. The second wiring is electrically connected to acircuit having a function of supplying a second potential. The thirdwiring is electrically connected to a circuit having a function ofsupplying a third potential. The fourth wiring is electrically connectedto a circuit having a function of supplying a fourth potential. Thesecond potential is lower than the fourth potential. The first potentialis used for controlling the amount of current flowing between the thirdwiring supplying the third potential and the fourth wiring supplying thefourth potential by the transistor.

In the semiconductor device according to one embodiment of the presentinvention, the switches are transistors.

In the semiconductor device according to one embodiment of the presentinvention, the switches are transistors and the transistors have thesame polarity.

In the semiconductor device according to one embodiment of the presentinvention, the load is preferably the display element with arectification property.

In one embodiment of the present invention, adverse effect of variationsin threshold voltage of transistors can be reduced. Alternatively,according to one embodiment of the present invention, adverse effect ofvariations in mobility of transistors can be reduced. Alternatively,according to one embodiment of the present invention, adverse effect ofdeterioration of a transistor can be reduced. Alternatively, accordingto one embodiment of the present invention, adverse effect ofdeterioration of a display element can be reduced. Alternatively,according to one embodiment of the present invention, display unevennesscan be reduced. Alternatively, according to one embodiment of thepresent invention, an image can be displayed with high display quality.Alternatively, according to one embodiment of the present invention, adesired circuit with a small number of transistors can be achieved.Alternatively, according to one embodiment of the present invention, adesired circuit with a small number of wirings can be achieved.Alternatively, one embodiment of the present invention can bemanufactured through a small number of steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are circuit diagrams each illustrating an example of acircuit of one embodiment of the present invention.

FIGS. 2A and 2B are circuit diagrams each illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 3A to 3C are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 4A to 4C are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 5A to 5D are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 6A to 6D are circuit diagrams each illustrating an example of asemiconductor device of one embodiment of the present invention.

FIG. 7 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 8 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 9 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 10 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIGS. 11A to 11D are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 12A to 12D are circuit diagrams each illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 13A and 13B are circuit diagrams each illustrating an example of asemiconductor device of one embodiment of the present invention.

FIG. 14 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 15 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 16 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIGS. 17A to 17C are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIGS. 18A to 18C are circuit diagrams each illustrating an example of asemiconductor device of one embodiment of the present invention.

FIG. 19 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 20 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 21 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 22 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIGS. 23A to 23C are circuit diagrams illustrating an example of asemiconductor device of one embodiment of the present invention.

FIG. 24 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 25 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 26 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 27 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIGS. 28A and 28B are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIGS. 29A and 29B are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIG. 30 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 31A to 31C are circuit diagrams illustrating an example of a pixelof one embodiment of the present invention.

FIGS. 32A to 32C are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIGS. 33A to 33D are circuit diagrams illustrating an example of a pixelof one embodiment of the present invention.

FIGS. 34A to 34D are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIG. 35 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 36 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 37 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 38 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 39 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 40A to 40D are circuit diagrams illustrating an example of a pixelof one embodiment of the present invention.

FIGS. 41A to 41D are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIGS. 42A and 42B are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIG. 43 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 44 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 45 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 46 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 47A to 47C are circuit diagrams illustrating an example of a pixelof one embodiment of the present invention.

FIGS. 48A to 48C are circuit diagrams each illustrating an example of apixel of one embodiment of the present invention.

FIG. 49 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 50 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 51 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 52 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 53 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 54A to 54C are circuit diagrams illustrating an example of a pixelof one embodiment of the present invention.

FIG. 55 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 56 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 57 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 58 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 59 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 60 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 61 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 62 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 63 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 64 is a top view illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 65 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 66 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 67 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 68 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 69 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 70 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 71A to 71E each illustrate a structure of an oxide material of oneembodiment of the present invention.

FIGS. 72A to 72C illustrate a structure of an oxide material of oneembodiment of the present invention.

FIGS. 73A to 73C are views illustrating a structure of an oxide materialof one embodiment of the present invention.

FIGS. 74A and 74B are views each illustrating a structure of an oxidematerial of one embodiment of the present invention.

FIG. 75A is a top view illustrating an example of a display panel cellof one embodiment of the present invention and FIG. 75B is across-sectional view illustrating an example of a display panel cell ofone embodiment of the present invention.

FIGS. 76A to 76H are diagrams each illustrating an electronic device towhich a display device of one embodiment of the present invention can beapplied.

FIGS. 77A to 77H are diagrams each illustrating an electronic device towhich a display device of one embodiment of the present invention can beapplied.

FIG. 78 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 79 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 80 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 81 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 82 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 83 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 84 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 85 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 86 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 87 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 88 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 89 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 90 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIGS. 91A and 91B are cross-sectional views each illustrating an exampleof a pixel of one embodiment of the present invention.

FIGS. 92A and 92B are cross-sectional views each illustrating an exampleof a pixel of one embodiment of the present invention.

FIG. 93 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 94 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 95 is a circuit diagram illustrating an example of a semiconductordevice of one embodiment of the present invention.

FIG. 96 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 97 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 98 is a circuit diagram illustrating an example of a pixel of oneembodiment of the present invention.

FIG. 99 is a diagram of a display module of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that a variety of changesand modifications can be made without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be limited to the descriptions of the embodiments below. Note thatin the structures described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals in commonin different drawings and repetitive description thereof will beomitted.

Note that what is described (or part thereof) in one embodiment can beapplied to, combined with, or exchanged with another content in the sameembodiment and/or what is described (or part thereof) in anotherembodiment or other embodiments.

Note that the structure of a diagram (or part of the diagram)illustrated in one embodiment can be combined with the structure ofanother part of the diagram, the structure of a different diagram (orpart of the different diagram) illustrated in the embodiment, and/or thestructure of a diagram (or part of the diagram) illustrated in one ormore different embodiments.

Note that the size, the thickness, or regions in diagrams are sometimesexaggerated for simplicity. Thus, one aspect of one embodiment of thepresent invention is not limited to such scales. Alternatively, thedrawings are perspective views of ideal examples. Thus, one aspect ofone embodiment of the present invention is not limited to shapes and thelike illustrated in the drawings. For example, the drawings can includevariations in shape due to a manufacturing technique or dimensionaldeviation.

Note that an explicit description “X and Y are connected” indicates thecase where X and Y are electrically connected, the case where X and Yare connected in terms of the function, the case where X and Y aredirectly connected, or the like. Here, each of X and Y denotes an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, a layer, or the like). Accordingly, anotherconnection relation shown in drawings and texts is included withoutbeing limited to a predetermined connection relation, for example, theconnection relation shown in the drawings and the texts.

For example, in the case where X and Y are electrically connected, oneor more elements which enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, and a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, the switch has a function of determiningwhether current flows or not by being turning on or off (becoming an onstate and an off state). Alternatively, the switch has a function ofdetermining and changing a current path. For example, the switch has afunction of determining whether current flows through a current path 1or a current path 2 and switching the paths.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a dc-dcconverter, a step-up dc-dc converter, or a step-down dc-dc converter) ora level shifter circuit for changing the potential level of a signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit that can increase signal amplitude, the amountof current, or the like, an operational amplifier, a differentialamplifier circuit, a source follower circuit, or a buffer circuit; asignal generation circuit; a memory circuit; and/or a control circuit)can be connected between X and Y. Note that for example, in the casewhere a signal output from X is transmitted to Y even when anothercircuit is interposed between X and Y, X and Y are functionallyconnected.

Note that an explicit expression “X and Y are electrically connected”means that X and Y are electrically connected, X and Y are functionallyconnected, and X and Y are directly connected. That is, when it isexplicitly described that “A and B are electrically connected”, thedescription is the same as the case where it is explicitly onlydescribed that “A and B are connected”.

Note that even when independent components are electrically connected toeach other in a circuit diagram, there is the case where one conductivelayer has functions of a plurality of components (e.g., a wiring and anelectrode), such as the case where part of a wiring functions as anelectrode. The expression “electrically connected” in this specificationalso means that one conductive layer has functions of a plurality ofcomponents.

Note that it might be possible for those skilled in the art toconstitute one embodiment of the invention even when portions to whichall terminals of an active element (e.g., a transistor or a diode), apassive element (e.g., a capacitor or a resistor), or the like areconnected are not specified. In particular, in the case where the numberof portions to which the terminal is connected might be plural, it isnot necessary to specify the portions to which the terminal isconnected. Therefore, it might be possible to constitute one embodimentof the invention by specifying only portions to which some of terminalsof an active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), or the like are connected.

Note that it might be possible for those skilled in the art to specifythe invention when at least a connection portion of a circuit isspecified. Alternatively, it might be possible for those skilled in theart to specify the invention when at least a function of a circuit isspecified. Therefore, when a connection portion of a circuit isspecified, the circuit is disclosed as one embodiment of the inventioneven when a function is not specified, and one embodiment of theinvention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Note that various people can implement the present invention describedin this specification and the like. However, different people mayimplement the present invention in a joint effort with each other. Forexample, when an invention relating to a transmission/reception systemis impremented, Company A manufactures and sells transmitting devices,and Company B manufactures and sells receiving devices, in some cases.As another example, when an invention relating to a light emittingdevice including TFTs and light-emitting elements is impremented,Company A manufactures and sells semiconductor devices including TFTsand Company B buys the semiconductor devices, deposits light-emittingelements to the semiconductor devices, and completes light emittingdevices, in some cases.

In such a case, with one embodiment of the present invention, a personcan file a patent infringement suit against Company A and Company B.That is, one embodiment of the present invention with which a person canfile a patent infringement suit against Company A and Company B is clearand regarded as being described in this specification or the like. Forexample, in the case of a transmission/reception system, one embodimentof the present invention can be constituted of only a transmittingdevice and can be constituted of only a receiving device. The embodimentof the present invention is clear and regarded as being described inthis specification or the like. As another example, in the case of alight emitting device including a TFT and a light-emitting element, oneembodiment of the present invention can be constituted of only asemiconductor device including a TFT and can be constituted of only alight emitting device including a TFT and a light-emitting element. Theembodiment of the present invention is clear and regarded as beingdescribed in this specification or the like.

The invention excluding content which is not specified in the drawingsand texts in this specification can be constituted. Alternatively, whenthe range of a value (e.g., the maximum and minimum values) isdescribed, part of the range is arbitrarily shortened and part of therange is removed, so that an invention can be specified by a range partof which is removed from the number range. In this manner, it ispossible to specify the scope of the present invention so that aconventional technology is excluded, for example.

As a specific example, a diagram of a circuit including a firsttransistor to a fifth transistor is illustrated. In that case, it can bespecified that a circuit of an invention does not include a sixthtransistor. Alternatively, it can be specified that a circuit of aninvention does not include a capacitor. Further alternatively, it can bespecified that a circuit of an invention does not include a sixthtransistor with a particular connection relation. Still alternatively,it can be specified that a circuit of an invention does not include acapacitor with a particular connection relation. For example, it can bespecified that a circuit of an invention does not include the sixthtransistor whose gate is connected to a gate of the third transistor.Alternatively, it can be specified that a circuit of an invention doesnot include the capacitor whose first electrode is connected to the gateof the third transistor.

As another specific example, a description says that “a voltage ispreferably higher than or equal to 3 V and lower than or equal to 10 V”.In that case, for example, it can be specified that an inventionexcludes the case where the voltage is higher than or equal to −2 V andlower than or equal to 1 V. Alternatively, for example, it can bespecified that an invention excludes the case where the voltage ishigher than or equal to 13V. Note that for example, it can be specifiedthat in an invention, the voltage is higher than or equal to 5 V andlower than or equal to 8 V. Note that for example, it can be specifiedthat in an invention, the voltage is approximately 9 V. Note that forexample, it can be specified that in an invention, the voltage is higherthan or equal to 3 V and lower than 9 V and higher than 9V and lowerthan or equal to 10 V.

As another specific example, a description says that “a voltage ispreferably 10 V”. In that case, for example, it can be specified that aninvention excludes the case where the voltage is higher than or equal to−2 V and lower than or equal to 1 V. Alternatively, for example, it canbe specified that an invention excludes the case where the voltage ishigher than or equal to 13 V.

As another specific example, a description says about a property of afilm that “the film is an insulating film”. In that case, for example,it can be specified that in an invention, the insulating film is not anorganic insulating film. Alternatively, for example, it can be specifiedthat in an invention, the insulating film is not an organic insulatingfilm.

As another specific example, a description says about a stackedstructure that “a film is provided between A and B”. In that case, forexample, it can be specified that in an invention, the film is not astacked film of four or more layers. Alternatively, for example, it canbe specified that in an invention, a conductive film is not providedbetween A and the film.

Embodiment 1

An embodiment of the present invention can be used for not only a pixelincluding a light-emitting element but also a variety of analog circuitsfunctioning as current sources. First, in this embodiment, examples of abasic principle of a circuit disclosed in the present invention isdescribed.

First, FIG. 1A illustrates a circuit configuration of one embodiment ofthe present invention. A semiconductor device 10 functions as at least acurrent source, for example. Accordingly, for example, the semiconductordevice 10 has a function of supplying a constant current even when thelevel of voltage applied to a wiring 20 and terminals of a load 16 ischanged. For example, the semiconductor device 10 can supply a constantcurrent to the load 16 even when the potential of the load 16 ischanged.

Note that there is a voltage source as a power source different from acurrent source. The voltage source has a function of supplying aconstant voltage even when current flowing through a circuit connectedto the voltage source is changed. Accordingly, the voltage source andthe current source both have a similar function. However, the voltagesource and the current source are different. Specifically, the voltagesource and the current source are different in what they supply, whichis voltage or current. Further, the voltage source and the currentsource are different. Specifically, the voltage source and the currentsource are different in parameter, change of which enables the voltagesource or the current source to supply voltage or current. The currentsource has a function of supplying a constant current event when voltagebetween both ends is changed. The voltage source has a function ofsupplying a constant voltage even when current is changed.

The circuit configuration illustrated in FIG. 1A has a circuit fordischarging electric charge held in a gate of a transistor in order tocorrect variations in current characteristics due to variations inthreshold voltage of the transistor or the like. In practice, thecircuit has a connection relation so that variations in currentcharacteristics of the transistor can be corrected by controlling theswitching of a plurality of switches provided between wirings.

The semiconductor device 10 illustrated in FIG. 1A includes a switch 12,a switch 13, a switch 14, a switch 15, a capacitor 17, and a transistor11 which allows the semiconductor device 10 to operate as a currentsource. The semiconductor device 10 is connected to the load 16, awiring 18, and the wiring 20. The load 16 is connected to a wiring 19.Note that in this embodiment, the transistor 11 which allows thesemiconductor device to operate as a current source is an n-channeltransistor, for example.

Next, a connection relation of components of the semiconductor device 10is described.

A gate of the transistor 11 is connected to one of electrodes(terminals) of the capacitor 17 and one of terminals of the switch 13. Afirst terminal (one of a source and a drain) of the transistor 11 isconnected to one of terminals of the switch 12 and one of terminals ofthe switch 14. A second terminal (the other of the source and the drain)of the transistor 11 is connected to the other of the terminals of theswitch 13 and one of terminals of the switch 15. Note that one ofterminals is also referred to as a first terminal and the other of theterminals is also referred to as a second terminal.

The other of the terminals of the switch 12 is connected to the wiring18.

The other of the terminals of the switch 15 is connected to the wiring20.

One of terminals of the load 16 is connected to the other of theterminals of the switch 14 and the other of the electrodes (terminals)of the capacitor 17. The other of the terminals of the load 16 isconnected to the wiring 19.

Note that in this specification, a semiconductor device means any devicewhich can function by utilizing semiconductor characteristics; a lightemitting device, a display device, a semiconductor circuit, and anelectronic device are included as examples of the semiconductor devicein some cases.

Note that in this specification, a load means an object having arectifying property, an object having a capacitive property, an objecthaving a resistive property, a circuit including a switch, a pixelcircuit, or the like. For example, the object having a rectifyingproperty has current-voltage characteristics showing differentresistance values based on the direction of an applied bias, and has anelectric property which allows current to flow only in one direction. Inthe circuit configuration illustrated in FIG. 1A, for example, the load16 is provided so that current flows from the transistor 11 to thewiring 19.

Alternatively, other examples of the load 16 are a display element(liquid crystal element), a light-emitting element (an EL element), andpart of a display element or a light-emitting element (e.g., a pixelelectrode, an anode electrode, and a cathode electrode).

Note that a transistor is an element having at least three terminals: agate, a drain, and a source. In addition, the transistor has a channelregion between a drain (drain terminal, a drain region, or a drainelectrode) and a source (source terminal, a source region, or a sourceelectrode), and current can flow through the drain, the channel region,and the source. Here, since the source and the drain of the transistormay change depending on the structure, the operating condition, and thelike of the transistor, it is difficult to define which is a source or adrain. Therefore, in this document (the specification, the claims, thedrawings, and the like), a region functioning as a source and a drain isnot called the source or the drain in some cases. In such a case, forexample, one of the source and the drain may be referred to as a firstterminal and the other thereof may be referred to as a second terminal.Alternatively, one of the source and the drain may be referred to as afirst electrode and the other thereof may be referred to as a secondelectrode. Further alternatively, one of the source and the drain may bereferred to as a first region and the other thereof may be referred toas a second region. Still alternatively, one of the source and the drainmay be referred to as a source region and the other thereof may becalled a drain region.

Note that terms such as “first”, “second”, “third”, and the like areused for distinguishing various elements, members, regions, layers, andareas from others. Therefore, the terms such as “first”, “second”,“third”, and the like do not limit the number of the elements, members,regions, layers, areas, or the like. Further, for example, “first” canbe replaced with “second”, “third”, or the like.

Note that the switch has a function of operating by bringing terminalsinto a conduction state (ON) or a non-conduction state (OFF) and afunction of determining whether or not current flows. The switch can bean electrical switch, a mechanical switch, or the like. For example, theswitch can be formed using a transistor, a diode, and a micro electromechanical system (MEMS) technology similarly to a digital micromirrordevice (DMD). Alternatively, the switch may be a logic circuit in whichtransistors are combined. Note that in the case of using a transistor, apolarity (conductivity type) thereof is not particularly limited. Notethat a transistor having a low off-state current is preferably used anda configuration in which the transistor has an appropriate polarity inaccordance with an input potential is preferable.

Examples of a transistor with lower off-state current are a transistorprovided with an LDD region, a transistor with a multi-gate structure,and a transistor including an oxide semiconductor in a semiconductorlayer. Alternatively in the case where a combination of transistorsfunctions as a switch, a complementary switch may be employed by usingboth n-channel and p-channel transistors. A complementary switchachieves an appropriate operation even when a potential input to theswitch is relatively changed in comparison with an output potential.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source and a drain), an output terminal (theother of the source and the drain), and a terminal for controllingconduction (gate) in some cases. On the other hand, when a diode is usedas a switch, the switch does not have a terminal for controllingelectrical conduction in some cases. Therefore, when a diode is used asa switch, the number of wirings for controlling terminals can be reducedas compared to the case of using a transistor.

Note that for example, a transistor with a structure where gateelectrodes are formed above and below a channel can be used as atransistor. With the structure where the gate electrodes are formedabove and below the channel, a circuit structure where a plurality oftransistors are connected in parallel is provided. Thus, a channelregion is increased, so that the amount of current can be increased.Alternatively, by employing the structure where gate electrodes areformed above and below the channel, a depletion layer is easily formed;thus, a subthreshold swing (an S value) can be reduced.

Note that for example, a transistor with a structure where a sourceelectrode or a drain electrode overlaps with a channel region (or partof it) can be used as a transistor. By using the structure where thesource electrode or the drain electrode may overlap with the channelregion (or part of it), an unstable operation due to electric chargeaccumulated in part of the channel region can be prevented.

Note that the capacitor 17 may have a structure in which an insulatingfilm is sandwiched between wirings, semiconductor layers, electrodes, orthe like, for example. The capacitor 17 has a function of capable ofholding voltage in accordance with characteristics of the transistor 11(e.g., voltage in accordance with a threshold voltage and a voltage inaccordance with mobility). Alternatively, the capacitor 17 has afunction of capable of holding voltage (e.g., a video signal) inaccordance with the amount of current supplied to the load 16.

Note that as illustrated in FIG. 1B, the wiring 18 is connected to acircuit 21 having at least a function of supplying Vinit or Vsig byswitching Vinit and Vsig, for example. An example of the circuit 21 is asource driver (signal line driver circuit). Accordingly, the wiring 18has a function of capable of transmitting or supplying Vinit and/orVsig.

The potential Vinit initializes the potential of each node in thesemiconductor device, for example. For example, Vinit is supplied beforeVsig which is a signal for making current flow to the load 16.

An example of Vsig is a signal for controlling the amount of currentflowing to the load 16. Therefore, a potential to be supplied depends onthe amount of current to be supplied to the load 16. For example, whencurrent supplied to the load 16 is constant, Vsig is a signal with aconstant potential. When current supplied to the load 16 is notconstant, Vsig is a signal with a potential which changes over timedepending on the amount of current to be supplied to the load 16.

Note that as illustrated in FIG. 1B, the wiring 19 is connected to acircuit 22 having at least a function of supplying Vcat, for example. Anexample of the circuit 22 is a power supply circuit. Accordingly, thewiring 19 has a function of capable of transmitting or supplying Vcat.

The potential Vcat is set to make current flow from the first electrodeside of the load 16 to the second electrode side of the load 16 in aperiod in which current flows to the load 16.

Note that as illustrated in FIG. 1B, the wiring 20 is connected to atleast a circuit 23 for supplying VDD. An example of the circuit 23 is apower supply circuit. Accordingly, the wiring 20 has a function ofcapable of transmitting or supplying VDD. Alternatively, the wiring 20has a function of capable of supplying current to the transistor 11.Alternatively, the wiring 20 has a function of capable of supplyingcurrent to the load 16.

The potential VDD is set to make current flow from the first electrodeside of the load 16 to the second electrode side of the load 16 throughthe transistor 11. Therefore, for example, VDD is higher than Vcat.

Note that each of the switch 12, the switch 13, the switch 14, and theswitch 15 which are illustrated in FIG. 1A can be a transistor. Thus, asan example, FIG. 2A illustrates the case where an n-channel transistoris used as each of the switch 12, the switch 13, the switch 14, and theswitch 15. Note that components in common with those in FIG. 1A aredenoted by common reference numerals, and the description thereof isomitted. All of the transistors have the same polarity as illustrated inFIG. 2A, whereby the semiconductor device can be manufactured in a smallnumber of steps. Thus, the manufacturing cost can be reduced.

In FIG. 2A, a transistor 12T corresponds to the switch 12, a transistor13T corresponds to the switch 13, a transistor 14T corresponds to theswitch 14, and a transistor 15T corresponds to the switch 15.

A gate of the transistor 12T is connected to a wiring 31. A firstterminal of the transistor 12T is connected to the first terminal of thetransistor 11 and a first terminal of the transistor 14T. A secondterminal of the transistor 12T is connected to the wiring 18. Therefore,the transistor 12T is in a conduction state when the potential of thewiring 31 is at an H level, and the transistor 12T is in anon-conduction state when the potential of the wiring 31 is at an Llevel.

Further, a gate of the transistor 13T is connected to a wiring 32. Afirst terminal of the transistor 13T is connected to the gate of thetransistor 11 and the one of the electrodes of the capacitor 17. Asecond terminal of the transistor 13T is connected to a first terminalof the transistor 15T and the second terminal of the transistor 11.Therefore, the transistor 13T is in a conduction state when thepotential of the wiring 32 is at an H level, and the transistor 13T isin a non-conduction state when the potential of the wiring 32 is at an Llevel.

Furthermore, a gate of the transistor 14T is connected to a wiring 33. Afirst terminal of the transistor 14T is connected to the first terminalof the transistor 11 and the first terminal of the transistor 12T. Asecond terminal of the transistor 14T is connected to the firstelectrode of the load 16 and the other electrode of the capacitor 17.Therefore, the transistor 14T is in a conduction state when thepotential of the wiring 33 is at an H level, and the transistor 14T isin a non-conduction state when the potential of the wiring 33 is at an Llevel.

In addition, a gate of the transistor 15T is connected to a wiring 34.The first terminal of the transistor 15T is connected to the secondterminal of the transistor 11 and the second terminal of the transistor13T. A second terminal of the transistor 15T is connected to the wiring20. Therefore, the transistor 15T is in a conduction state when thepotential of the wiring 34 is at an H level, and the transistor 15T isin a non-conduction state when the potential of the wiring 34 is at an Llevel.

Note that for example, the wiring 31 is connected to a circuit 24A, thewiring 32 is connected to a circuit 24B, the wiring 33 is connected to acircuit 24C, and the wiring 34 is connected to a circuit 24D. Thecircuits 24A to 24D each have a function of supplying a signal at an Hlevel or an L level, for example. An example of each of the circuits 24Ato 24D is a gate driver (scan line driver circuit) or the like.Accordingly, the wiring 31 has a function of capable of transmitting orsupplying a signal at an H level or an L level. Alternatively, thewiring 31 has a function of capable of controlling a conduction state ofthe switch 12 or the transistor 12T. The wiring 32 has a function ofcapable of controlling a conduction state of the switch 13 or thetransistor 13T. The wiring 33 has a function of capable of controlling aconduction state of the switch 14 or the transistor 14T. The wiring 34has a function of capable of controlling a conduction state of theswitch 15 or the transistor 15T.

Note that the wiring 31, the wiring 32, the wiring 33, and the wiring 34can function as different wirings. However, one embodiment of thepresent invention is not limited thereto. The wirings 31 to 34 can becombined into one wiring; therefore, it is possible to form a circuitwith a small number of wirings.

For example, the wiring 31 and the wiring 32 can be combined into onewiring. Therefore, the wiring 31 can be connected to the wiring 32 to beone wiring. At this time, the transistor 12T and the transistor 13Tpreferably have the same polarity. FIG. 93 shows a circuit diagram ofthis case.

For example, the wiring 33 and the wiring 34 can be combined into onewiring. Therefore, the wiring 33 can be connected to the wiring 34 to beone wiring. At this time, the transistor 14T and the transistor 15Tpreferably have the same polarity. FIG. 94 shows a circuit diagram ofthis case.

Note that the wiring 31 and the wiring 32 can be combined into onewiring and the wiring 33 and the wiring 34 can be combined into onewiring. FIG. 95 shows a circuit diagram in that case.

In many cases, the transistor 11 operates in a saturation region at thetime of passing current. Therefore, the transistor 11 preferably has alonger channel length or gate length than the transistor 12T, thetransistor 13T, the transistor 14T, or the transistor 15T. When thechannel length or the gate length is longer, characteristics in asaturation region have a flat slope; accordingly, a kink effect can bereduced. Note that one embodiment of the present invention is notlimited to these examples.

In many cases, the transistor 11 operates in a saturation region at thetime of passing current. Therefore, the transistor 11 preferably has alarger channel width or gate width than the transistor 12T, thetransistor 13T, the transistor 14T, or the transistor 15T. When thechannel width or the gate width is larger, a large amount of current canflow even in a saturation region. Note that one embodiment of thepresent invention is not limited to these examples.

Next, the operation of the semiconductor device 10 illustrated in FIG.1A is described. The operation of the semiconductor device 10illustrated in FIG. 1A can be mainly divided into a first operation, asecond operation, and a third operation. Note that one embodiment of thepresent invention is not limited thereto, and another operation can beadded or part of the operation can be skipped.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 1A, FIG. 2B shows symbols representing thepotentials of nodes between elements and the potentials of wirings. FIG.2B also shows Vgs between the one of the terminals (mainly serving as asource) and the gate of the transistor 11 and Vc between the electrodesof the capacitor 17.

A nodeA, a nodeB, a nodeC, a nodeD, a nodeE, a nodeF, and a nodeGcorrespond to nodes and wirings illustrated in FIG. 2B. The potential ofthe nodeA corresponds to the potential of the wiring 18. The potentialof the nodeB corresponds to the potential of a wiring connecting thefirst terminal of the transistor 11, the first terminal of the switch12, and the first terminal of the switch 14. The potential of the nodeCcorresponds to the potential of a wiring connecting the second terminalof the switch 14, the one of the terminals of the load 16, and the otherof the electrodes of the capacitor 17. The potential of the nodeDcorresponds to the potential of the wiring 19. The potential of thenodeE corresponds to the potential of a wiring connecting the gate ofthe transistor 11, the one of electrodes of the capacitor 17, and thefirst terminal of the switch 13. The potential of the nodeF correspondsto the potential of a wiring connecting the second terminal of thetransistor 11, the second terminal of the switch 13, and the firstterminal of the switch 15. The potential of the nodeG corresponds to thepotential of the wiring 20.

First, the first operation is described with reference to FIG. 3A. Notethat reference numerals of elements in FIG. 3A are omitted. Note that inthe drawings, a conduction state and a non-conduction state of theswitches are denoted by ON and OFF. In addition, how Vgs, Vc, thepotential of the nodeA, the potential of the nodeB, the potential of thenodeC, the potential of the nodeD, the potential of the nodeE, thepotential of the nodeF, and the potential of the nodeG, which areillustrated in FIG. 2B, are applied is described.

The first operation initializes the potential of each node.Specifically, the nodeA is set at Vinit, the nodeD is set at Vcat, andthe nodeG is set at VDD. Then, the switch 12, the switch 13, the switch14, and the switch 15 are turned on. Thus, the nodeB is set at Vinit,the nodeC is set at Vinit, the nodeE is set at VDD, and the nodeF is setat VDD. Further, Vgs becomes (VDD−Vinit), and Vc becomes (VDD−Vinit).

As described above, in the first operation, Vinit at the nodeB and thenodeC is equal to or lower than Vcat at the nodeD, for example. Withthis structure, current is prevented from flowing to the load 16 in thefirst operation. Accordingly, problems caused by current flowing to theload 16 can be reduced. Further, when Vinit is lower than Vcat, the load16 can be reverse biased. In that case, deterioration of the load 16 canbe reduced and the load 16 can be repaired.

In the first operation, VDD at the nodeE and the nodeF is higher thanVcat at the nodeD. With this structure, Vgs can be higher than thethreshold voltage of the transistor 11 in the first operation.Alternatively, electric charge can be charged in the capacitor 17.

Next, the second operation is described with reference to FIG. 3B, as inFIG. 3A.

The second operation is the operation for obtaining the thresholdvoltage of the transistor 11 as Vgs by discharging the potential of thegate of the transistor 11 (or the electric charge charged in thecapacitor 17). Specifically, the nodeA is set at Vsig, the nodeD is setat Vcat, and the nodeG is set at VDD. Then, the switch 12 and the switch13 are turned on, and the switch 14 and the switch 15 are turned off.Accordingly, the potential of the nodeB becomes Vsig, the potential ofthe nodeC becomes (Vinit−Vx), the potential of the nodeE becomes(Vsig+Vth), and the potential of the nodeF becomes (Vsig+Vth). Further,Vgs becomes Vth and Vc becomes (Vsig+Vth−Vinit+Vx).

As described above, Vsig at the nodeB in the second operation is thepotential used for controlling the amount of current flowing between thewiring 20 and the wiring 19 with the use of the transistor 11 in thethird operation. By the second operation, the potential of the nodeEcorresponding to the potential of the gate of the transistor 11 can be(Vsig+Vth) which includes the threshold voltage of the transistor 11.

In the second operation, Vx of the potential of the nodeC (Vinit−Vx)changes when the nodeC is set in an electrically floating state. In thiscase, the amount of changes in Vx depends on a ratio of parasiticcapacitance of the load 16 to the capacitance of the capacitor 17. Notethat Vx is preferably set to a low potential in advance. Specifically,the parasitic capacitance of the load 16 is set to be sufficientlylarger than the capacitance of the capacitor 17, whereby Vx can be low.It is preferable that the parasitic capacitance of the load 16 be twotimes or more, more preferably four times or more the capacitance of thecapacitor 17.

Further, VDD at the nodeD and the nodeE in the first operation isdischarged by the second operation. By the discharging, Vgs is decreasedto the threshold voltage Vth of the transistor 11 and is set in a steadystate. Therefore, the discharging makes the nodeD and the nodeE are setin a steady state at (Vsig+Vth). In addition, at the time of terminatingthe second operation, (Vsig+Vth−Vinit+Vx) is held as Vc.

Note that in some cases, it takes a very long time until Vgs becomesequal to the threshold voltage Vth of the transistor 11. Accordingly, inmany case, the semiconductor device is driven while Vgs is notcompletely decreased to the threshold voltage Vth. That is, in manycases, the second operation is terminated while Vgs is slightly higherthan the threshold voltage Vth. In other words, at the time ofterminating the second operation, Vgs is based on the threshold voltage.

Note that in the second operation, the switch 14 and the switch 15 areturned off and the potential of the nodeB is set to Vsig. Theseoperations can be performed at the same time or at different timings.

It is preferable that, for example, the potential of the nodeB bechanged from Vinit to Vsig at the same time as or after the switch 14 isturned off. This is because the potential of the nodeC can be easilyheld at an appropriate potential.

Alternatively, it is preferable that, for example, the potential of thenodeB be changed from Vinit to Vsig before or at the same time as theswitch 15 is turned off. This is because the gate potential of thetransistor 101 can be quickly lowered.

Next, the third operation is described with reference to FIG. 3C, as inFIGS. 3A and 3B.

The third operation is the operation for outputting current to the load16 with the use of the transistor 11 as part of a current source.Specifically, the nodeA is set at Vsig, for example, though it can beany potential, the nodeD is set at Vcat, and the nodeG is set at VDD.Then, the switch 14 and the switch 15 are turned on, and the switch 12and the switch 13 are turned off. Accordingly, the nodeB and the nodeCbecome Vel, the nodeE becomes (Vsig+Vth−Vinit+Vx+Vel), and the nodeFbecomes VDD. In addition, Vgs becomes (Vsig+Vth−Vinit+Vx) and Vc becomes(Vsig+Vth−Vinit+Vx).

Note that in the third operation, the potentials of the nodeB, thenodeC, and the nodeF are increased while the nodeE is kept in anelectrically floating state. Accordingly, the potential of the nodeE isincreased by capacitive coupling while (Vsig+Vth−Vinit+Vx) is held asVc, thereby becoming (Vsig+Vth−Vinit+Vx+Vel). That is, an increase inthe potential of the nodeC leads to an increase in the potential of thenodeE by bootstrap operation.

The semiconductor device can operate even when the potential of thenodeC is increased; therefore, adverse effect of deterioration involtage current characteristics of the load (e.g., a display element anda light-emitting element) can be reduced even when the deterioration iscaused.

The potential Vel which is the potentials of the nodeB and the nodeC isset when the potential of the nodeF is increased to VDD and currentflows to the load 16 through the transistor 11 which allows thesemiconductor device to operate as a current source by the thirdoperation. Specifically, the potential ranges from VDD to Vcat.

In the third operation, Vgs of the transistor 11 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 11. The amount of current of the transistor 11 depends on(Vgs−Vth). Accordingly, through the above operations, adverse effect ofvariations in the threshold voltage of the transistor on the amount ofcurrent supplied to the load can be reduced. Alternatively, even whenthe threshold voltage is changed by deterioration of the transistor,adverse effect of the change can be reduced. Therefore, in the case of adisplay element, display unevenness can be reduced and display can beperformed with high quality.

Note that in the third operation, the switch 12 and the switch 13 areturned off and the switch 14 and the switch 15 are turned on. Theseoperations can be performed at the same time or at different timings.

For example, it is preferable that the switch 14 and the switch 15 beturned on after the switch 12 and the switch 13 are turned off. This isbecause Vc can be easily held at an appropriate potential.

Alternatively, for example, it is preferable that the switch 12 beturned off after the switch 13 is turned off. This is because Vc can beeasily held at an appropriate potential.

Note that FIG. 1A illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The locations of the switches or the number of switches can bechanged and/or appropriate voltage can be supplied so that theoperations become similar to the operations described in FIGS. 3A to 3Cin which the threshold voltage of the transistor is corrected. In such amanner, a variety of circuits can be employed.

For example, specifically, the switch 12, the switch 13, the switch 14,and the switch 15 can be provided at any place and the number ofswitches is not limited as long as the switches can control a conductionstate and a non-conduction state between nodes. In the case of the firstoperation described with reference to FIG. 3A, a connection relationillustrated in FIG. 4A may be employed. In the case of the secondoperation described with reference to FIG. 3B, a connection relationillustrated in FIG. 4B can be employed. In the case of the thirdoperation described with reference to FIG. 3C, a connection relationillustrated in FIG. 4C can be employed. The potential of each node canhave any level unless the node affects the operations.

Note that the operation for correcting the threshold voltage of thetransistor is described with reference to FIGS. 3A to 3C but oneembodiment of the present invention is not limited thereto. For example,the operation for correcting variations in the mobility of thetransistor 11 may be performed between the second operation in FIG. 3Band the third operation in FIG. 3C. FIGS. 5A to 5D illustrate the casewhere the operation for correcting variations in the mobility of thetransistor 11 is added to the first to third operations which aredescribed with reference to FIGS. 3A to 3C.

Note that a first operation illustrated in FIG. 5A is the same as thefirst operation described with reference to FIG. 3A; therefore, thedescription thereof is omitted. A second operation illustrated in FIG.5B is the same as the second operation described with reference to FIG.3B; therefore, the description thereof is omitted.

Next, a third operation is described with reference to FIG. 5C, as inFIGS. 3A and 3B.

In the third operation, the transistor 11 is turned on with the use ofthe potential held in the gate of the transistor 11 (electric chargestored in the capacitor 17), and the mobility of the transistor 11 iscorrected with the use of the amount of current flowing therethrough.Specifically, the nodeA is set at Vsig though it can be any potential,the nodeD is set at Vcat, and the nodeG is set at VDD though it can beany potential. Then, the switch 13 and the switch 14 are turned on, andthe switch 12 and the switch 15 are turned off. Then, the amount ofchange in potentials of the nodeB and the nodeC is −ΔVel, the nodeE andthe nodeF become (Vsig+Vth−ΔVel). In addition, Vgs becomes (Vth+ΔVel)and Vc becomes (Vsig+Vth−Vinit+Vx−ΔVel).

Note that in the third operation, the potentials of the nodeB and thenodeC are changed by turning on the switch 14. The amount of changes inthe potentials corresponds to −ΔVel. When the amount of change in thepotentials of the nodeB and the nodeC becomes −ΔVel, Vgs becomes(Vth+ΔVel) and higher than the threshold voltage Vth; as a result,current flows through the transistor 11. When current flows through thetransistor 11, each of the potentials of the nodeE and the nodeF isdecreased to (Vsig+Vth−ΔVel) and Vc becomes (Vsig+Vth−Vinit+Vx−ΔVel).

The amount of current flowing to the transistor 11 changes depending onthe mobility of the transistor 11. Accordingly, the potential of thenodeE corresponding to the gate of the transistor 11 can be set so as toinclude the amount of change in potential corresponding to the mobilityof the transistor 11.

In the third operation, the potential of the gate of the transistor 11becomes (Vsig+Vth−ΔVel) which is set in consideration of the mobility ofthe transistor 11. Accordingly, through the above operations, adverseeffect of variations in the mobility of the transistor on the amount ofcurrent supplied to the load can be reduced. Alternatively, even whenmobility is changed by deterioration of the transistor, adverse effectof the change can be reduced.

Next, a fourth operation is described with reference to FIG. 5D, as inFIGS. 3A and 3B. Note that the fourth operation illustrated in FIG. 5Dis similar to the third operation described with reference to FIG. 3C;therefore, only an aspect different from the third operation withreference to FIG. 3C is described.

By the fourth operation, the nodeB and the nodeC are set at Vel, thenodeE is set at (Vsig+Vth−Vinit+Vx−ΔVel+Vel), and the nodeF is set atVDD. The potential Vgs becomes (Vsig+Vth−Vinit+Vx−ΔVel) and Vc becomes(Vsig+Vth−Vinit+Vx).

In the fourth operation, Vgs of the transistor 11 becomes(Vsig+Vth−Vinit+Vx+ΔVel), which can be set in consideration of thethreshold voltage and the mobility of the transistor 11. Accordingly,through the above operations, adverse effect of variations in thethreshold voltage and the mobility of the transistor on the amount ofcurrent supplied to the load can be reduced.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIGS. 5A to 5D in which thethreshold voltage of the transistor is corrected. In such a manner, avariety of circuits can be employed.

For example, specifically, the switch 12, the switch 13, the switch 14,and the switch 15 can be provided any place and the numbers thereof isnot limited as long as the switches can control a conduction state and anon-conduction state between nodes. In the case of the first operationdescribed with reference to FIG. 5A, a connection relation illustratedin FIG. 6A can be employed. In the case of the second operationdescribed with reference to FIG. 5B, a connection relation illustratedin FIG. 6B can be employed. In the case of the third operation describedwith reference to FIG. 5C, a connection relation illustrated in FIG. 6Ccan be employed. In the case of the fourth operation described withreference to FIG. 5D, a connection relation illustrated in FIG. 6D canbe employed. The potential of each node can have any level unless thenode affects the operations.

Note that FIG. 1A illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The number of switches or the locations of the switches can bechanged and a variety of circuits can be employed.

For example, as in a semiconductor device 10A illustrated in FIG. 7, thetransistor 11A and the transistor 11B which have gates connected to eachother and which are connected in series can be used as transistors whichallow the semiconductor device to serve as a current source. Note thatcomponents in common with those in FIG. 1A are denoted by commonreference numerals, and the description thereof is omitted.

As another example, as in a semiconductor device 10B illustrated in FIG.8, the transistor 11A and the transistor 11B which have gates connectedto each other and which are connected in parallel can be used astransistors which allow the semiconductor device to serve as a currentsource. Note that components in common with those in FIG. 1A are denotedby common reference numerals, and the description thereof is omitted.

As another example, as in a semiconductor device 10C illustrated in FIG.9, the transistor 11A, the transistor 11B, the transistor 11C, and thetransistor 11D which have gates connected to each other and which areconnected in series and parallel can be used as transistors which allowthe semiconductor device to serve as a current source. Note thatcomponents in common with those in FIG. 1A are denoted by commonreference numerals, and the description thereof is omitted.

The channel width and/or the channel length of the transistor 11 can bechanged by application of the structures illustrated in FIG. 7, FIG. 8,and FIG. 9. With the structures illustrated in FIG. 7, FIG. 8, and FIG.9 in which channel widths and/or channel lengths of a plurality oftransistors can be changed after the transistors are combined, adverseeffect of variations in characteristics of the transistors can besmaller in comparison with the structure in which transistors eachhaving a large channel width and/or a large channel length is providedin advance.

Note that FIG. 1A, FIG. 2A, or the like illustrates an example of acircuit configuration; accordingly, a transistor can be providedadditionally. On the other hand, in each node in FIG. 1A, FIG. 2A, orthe like, it is also possible not to provide an additional transistor,switch, passive element, or the like. For example, it is possible not toincrease the number of transistors directly connected to the nodeA, thenodeB, the nodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG.Accordingly, for example, the following structure can be used: only thetransistor 14T is directly connected to the nodeC and the othertransistors are not directly connected to the nodeC.

Therefore, a circuit can be formed with a small number of transistors inthe case where a transistor is not added.

Note that variations in the threshold voltage or the like of atransistor is corrected in this embodiment, but one embodiment of thepresent invention is not limited thereto. For example, current can besupplied to the load 16 and the semiconductor device can be drivenwithout performing the operation for correcting variations in thresholdvoltage.

This embodiment shows an example of a basic principle. Thus, part of orthe whole of this embodiment can be freely combined with, applied to, orreplaced with part of or the whole of another embodiment.

Embodiment 2

In this embodiment, an example of a configuration different from thecircuit configuration of the semiconductor device described inEmbodiment 1 is described.

FIG. 10 illustrates a semiconductor device 10 h having a circuitconfiguration similar to the semiconductor device 10 illustrated in FIG.1A. The semiconductor device 10 h illustrated in FIG. 10 is differentfrom the semiconductor device 10 illustrated in FIG. 1A in that thesemiconductor device 10 h is connected to a circuit 21 h and a circuit23 h. The circuit 21 h has at least a function of supplying Vsig andVinit to the wiring 18, and the circuit 23 h has at least a function ofsupplying Vinit or VDD to the wiring 20, switching them as necessary.Note that components in common with those in FIG. 1A are denoted bycommon reference numerals, and the description thereof is omitted. Anexample of the circuit 21 h is a source driver (signal line drivercircuit). Examples of the circuit 23 h are a gate driver (scan linedriver circuit) and a power supply circuit.

Next, the operation of the semiconductor device 10 h illustrated in FIG.10 is described. The operation of the semiconductor device 10 hillustrated in FIG. 10 can be mainly divided into a first operation, asecond operation, a third operation, and a fourth operation. Oneoperation is added to the operations of the semiconductor device 10illustrated in FIG. 1A or the like. The second operation, the thirdoperation, and the fourth operation of the semiconductor device 10 hillustrated in FIG. 10 correspond to the first operation, the secondoperation, and the third operation of the semiconductor device 10illustrated in FIG. 1A, respectively.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 10, FIG. 10 shows symbols representing thepotentials of nodes between elements and the potentials of wirings, asFIG. 2B does. The operation of the circuit configuration illustrated inFIG. 10 is explained with symbols of Vgs and Vc, as in FIG. 2B.

In the first operation, a potential for initialization is applied tosome extent at each node before initialization of the potential of eachnode (initialization before initialization). Specifically, the nodeG isset at Vinit and the nodeD is set at Vcat. The nodeA can be set at anypotential. In addition, the switch 14 and the switch 15 are turned on,and the switch 12 and the switch 13 are turned off. The nodeB and thenodeC are then set at Vinit or the potential ΔVinit which is close toVinit. The nodeE is set at Vy and the nodeF is set at Vinit. Note thatVgs and Vc are omitted because the first operation uses Vy which is asignal of an operation before the first operation.

The potential Vy is input before the first operation. The case where Vyenables the transistor 11 to operate as part of a current source isexplained. The potential Vy is set so that current flows between thefirst terminal and the second terminal of the transistor 11 in the firstoperation. Usually, Vinit is very low and accordingly the transistor 11is turned on because of Vy in many cases.

Therefore, in the first operation, the nodeF is set at Vinit and currentflows between the first terminal and the second terminal of thetransistor 11; as a result, the nodeB and the nodeC are set at Vinit orthe potential ΔVinit which is close to Vinit.

That is, the first operation decreases the potentials of the nodeB andthe nodeC. By the decrease in the potentials of the nodeB and the nodeCin the first operation, the following second operation can initializethe potential of each node at high speed. In particular, when the load16 has large capacitance, the following operation can be performedsmoothly by the decrease in the potentials of the nodeB and the nodeC inadvance. Note that even if the potentials of the nodeB and the nodeCcannot be sufficiently decreased, it is not a problem unless subsequentoperations are adversely affected.

The second operation is the same as the first operation described withreference to FIG. 3A and therefore the description thereof is omitted.

In the second operation, the switch 12 and the switch 13 are turned onand the potential of the nodeG is set to VDD, and these operations canbe performed at the same time or at different timings.

It is preferable that, for example, the potential of the nodeG bechanged from Vinit to VDD before or at the same time as the switch 13 isturned on. This is because the potential of the nodeE can be increasedeasily in that case.

Then, the third operation illustrated in FIG. 11C is the same as thesecond operation described with reference to FIG. 3B and therefore thedescription thereof is omitted. Then, the fourth operation illustratedin FIG. 11D is the same as the third operation described with referenceto FIG. 3C and therefore the description thereof is omitted.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIGS. 11A to 11D in which thethreshold voltage of the transistor is corrected. In such a manner, avariety of circuits can be employed.

For example, specifically, the switch 12, the switch 13, the switch 14,and the switch 15 can be provided any place and the numbers thereof isnot limited as long as the switches can control a conduction state and anon-conduction state between nodes. In the case of the first operationdescribed with reference to FIG. 11A, a connection relation illustratedin FIG. 12A can be employed. In the case of the second operationdescribed with reference to FIG. 11B, a connection relation illustratedin FIG. 12B can be employed. In the case of the third operationdescribed with reference to FIG. 11C, a connection relation illustratedin FIG. 12C can be employed. In the case of the fourth operationdescribed with reference to FIG. 11D, a connection relation illustratedin FIG. 12D can be employed. The potential of each node can have anylevel unless the node affects the operations.

Note that the operation for correcting the threshold voltage of thetransistor is described with reference to FIGS. 11A to 11D but oneembodiment of the present invention is not limited thereto. For example,the operation for correcting variations in the mobility of thetransistor 11 may be performed between the third operation in FIG. 11Cand the fourth operation in FIG. 11D.

The operation for correcting the mobility of the transistor 11 isdescribed with reference to FIG. 13A.

The operation for correcting the mobility of the transistor 11 is thesame as the third operation described with reference to FIG. 5C and thedescription thereof is omitted.

In the operation for correcting the mobility of the transistor 11, thepotential of the gate of the transistor 11 becomes (Vsig+Vth−ΔVel) whichis set in consideration of the mobility of the transistor 11.Accordingly, through the above operation, adverse effect of variationsin the mobility of the transistor on the amount of current supplied tothe load can be reduced.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIG. 13A in which the mobilityof the transistor is corrected. In such a manner, a variety of circuitscan be employed.

For example, specifically, the switch 12, the switch 13, the switch 14,and the switch 15 can be provided any place and the numbers thereof isnot limited as long as the switches can control a conduction state and anon-conduction state between nodes. In the case of the operation forcorrecting the mobility of the transistor described with reference toFIG. 13A, a connection relation illustrated in FIG. 13B can be employed.The potential of each node can have any level unless the node affectsthe operations.

The potential of the wiring 20 is switched between Vinit and VDD in thecircuit configuration illustrated in FIG. 10, but another configurationcan be used. For example, a configuration illustrated in FIG. 14 may beemployed: a wiring 20A and a wiring 20B are provided instead of thewiring 20, and Vinit is supplied from a circuit 23A connected to thewiring 20A and VDD is supplied from a circuit 23B connected to thewiring 20B. At this time, a switch 15A provided between the wiring 20Aand the nodeF and a switch 15B provided between the wiring 20B and thenodeF may perform switching so as to achieve the similar operation tothe operation described with reference to FIGS. 11A to 11D. That is, thecircuit 23A has a function of supplying Vinit and examples of thecircuit 23A are a power supply circuit and a voltage follower circuit.The circuit 23B has a function of supplying VDD and an example of thecircuit 23B is a power supply circuit. In addition, the switching of theswitch 15A is controlled with a wiring 34A and the switching of theswitch 15B is controlled with a wiring 34B. As an example, the wiring34A and the wiring 34B are connected to a circuit 25A and a circuit 25B,respectively. The circuit 25A and the circuit 25B each have at least afunction of supplying an H-level signal or an L-level signal. An exampleof each of the circuit 25A and the circuit 25B is a gate driver (scanline driver circuit).

As described above, in the circuit configuration described in thisembodiment, initialization before initialization can be performed byswitching of the potential of the wiring 20 between Vinit and VDD.Accordingly, the potential of each node can be initialized at highspeed. In the fourth operation, Vgs of the transistor 11 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 11. Accordingly, with this structure, adverse effect ofvariations in the threshold voltage of the transistor on the amount ofcurrent supplied to the load can be reduced.

Note that FIG. 14, or the like illustrates an example of a circuitconfiguration; accordingly, a transistor can be provided additionally.In each node in FIG. 14, or the like, it is possible not to provide anadditional transistor, switch, a passive element, or the like. Forexample, transistors directly connected to the nodeA, the nodeB, thenodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG are notadditionally provided. Accordingly, for example, the following structurecan be used: only the transistor 14T is directly connected to the nodeCand the other transistors are not directly connected to the nodeC.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 3

In this embodiment, an example of a configuration different from thecircuit configurations of the semiconductor devices described inEmbodiments 1 and 2 is described.

FIG. 15 illustrates a semiconductor device 10 p having a circuitconfiguration similar to the semiconductor device 10 illustrated in FIG.1A. The semiconductor device 10 p illustrated in FIG. 15 is differentfrom the semiconductor device 10 illustrated in FIG. 1A in that thepotential supplied to the wiring 18 is Vsig, a wiring 18 p and a switch12 p are provided, and Vinit is supplied from the wiring 18 p. Note thatcomponents in common with those in FIG. 1A are denoted by commonreference numerals, and the description thereof is omitted.

In FIG. 15, a first terminal of the switch 12 p is connected to thefirst terminal of the transistor 11, the first terminal of the switch12, and the first terminal of the switch 14. A second terminal of theswitch 12 p is connected to the wiring 18 p.

Next, the operation of the semiconductor device 10 h illustrated in FIG.15 is described. The operation of the semiconductor device 10 hillustrated in FIG. 15 can be mainly divided into the first operation,the second operation, and the third operation.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 15, FIG. 16 shows symbols representing thepotentials of nodes between elements and the potentials of wirings. FIG.16 also shows Vgs between the one of the terminals (mainly serving as asource) and the gate of the transistor 11 and Vc between the electrodesof the capacitor 17. The switching of the switch 12 p is controlled witha wiring 31 p. In FIG. 16, a circuit 26 is connected to the wiring 31 pand has at least a function of supplying an H-level signal or an L-levelsignal. An example of the circuit 26 is a gate driver (scan line drivercircuit). Further, in FIG. 16, a circuit 21 p is connected to the wiring18 p and has a function of supplying Vinit to the wiring 18 p. Anexample of the circuit 21 p is a power supply circuit and a voltagefollower circuit.

A nodeA, a nodeB, a nodeC, a nodeD, a nodeE, a nodeF, a nodeG, and anodeH correspond to nodes and wirings illustrated in FIG. 16. Thepotential of the nodeA corresponds to the potential of the wiring 18.The potential of the nodeB corresponds to the potential of a wiringconnecting the first terminal of the transistor 11, the first terminalof the switch 12, the first terminal of the switch 14, and the firstterminal of the switch 12 p. The potential of the nodeC corresponds tothe potential of a wiring connecting the second terminal of the switch14, the one of the terminals of the load 16, and the other of theelectrodes of the capacitor 17. The potential of the nodeD correspondsto the potential of the wiring 19. The potential of the nodeEcorresponds to the potential of a wiring connecting the gate of thetransistor 11, the one of electrodes of the capacitor 17, and the firstterminal of the switch 13. The potential of the nodeF corresponds to thepotential of a wiring connecting the second terminal of the transistor11, the second terminal of the switch 13, and the first terminal of theswitch 15. The potential of the nodeG corresponds to the potential ofthe wiring 20. The potential of the nodeE the nodeH corresponds to thepotential of the wiring 18 p.

First, the first operation is described with reference to FIG. 17A. Notethat reference numerals of elements in FIG. 17A are omitted. Aconduction state and a non-conduction state of the switches are denotedby ON and OFF. In addition, how Vgs, Vc, the potential of the nodeA, thepotential of the nodeB, the potential of the nodeC, the potential of thenodeD, the potential of the nodeE, the potential of the nodeF, thepotential of the nodeG, and the potential of the nodeH, which areillustrated in FIG. 16, are applied is described.

The first operation initializes the potential of each node.Specifically, the nodeA is set at any potential, the nodeD is set atVcat, the nodeG is set at VDD, and the nodeH is set at Vinit. Then, theswitch 12 p, the switch 13, the switch 14, and the switch 15 are turnedon, and the switch 12 is turned off. Thus, the nodeB is set at Vinit,the nodeC is set at Vinit, the nodeE is set at VDD, and the nodeF is setat VDD. Further, Vgs becomes (VDD−Vinit), and Vc becomes (VDD−Vinit).

The first operation described with reference to FIG. 17A is differentfrom that of described with reference to FIG. 3A in Embodiment 1 in thatVinit supplied to the nodeB and the nodeC is supplied from the wiring 18p through the switch 12 p. With the structure, initialization can beperformed without change in potential of the wiring 18 and theinitialization of each node can be performed at high speed.Alternatively, initialization of each node of the above semiconductordevice 10 p can be performed while a potential is supplied from thewiring 18 to another semiconductor device 10 p connected to the wiring18. Therefore, an operation period for the initialization can be longer.

Next, the second operation is described with reference to FIG. 17B, asin FIG. 17A.

The second operation is the operation for obtaining the thresholdvoltage of the transistor 11 with the use of Vgs by discharging thepotential of the gate of the transistor 11 (or the electric charge ofthe capacitor 17). Specifically, the nodeA is set at Vsig, the nodeD isset at Vcat, the nodeG is set at VDD, and the nodeH is set at Vinitthough it can be any potential. Then, the switch 12 and the switch 13are turned on, and the switch 14, the switch 12 p, and the switch 15 areturned off. Thus, the potential of the nodeB becomes Vsig, the potentialof the nodeC becomes (Vinit−Vx), the potential of the nodeE becomes(Vsig+Vth), and the potential of the nodeF becomes (Vsig+Vth). Further,Vgs becomes Vth and Vc becomes (Vsig+Vth−Vinit+Vx).

The second operation described with reference to FIG. 17B is differentfrom that described in Embodiment 1 with reference to FIG. 3B in thatthe switch 12 p is turned off. Therefore, the second operation in thisembodiment is the same as the second operation described with referenceto FIG. 3B. By the second operation, the potential of the nodeEcorresponding to the potential of the gate of the transistor 11 can be(Vsig+Vth) which includes the threshold voltage of the transistor 11.

Note that in the second operation, the switch 14, the switch 15, and theswitch 12 p are turned off and the switch 12 is turned on, and theseoperations can be performed at the same time or at different timings.

For example, it is preferable that the switch 12 be turned on at thesame time as or after the switch 12 p is turned off. This is because ashort circuit between the nodeA and the nodeH can be prevented easily.

Next, the third operation is described with reference to FIG. 17C, as inFIGS. 17A and 17B.

The third operation is the operation for outputting current to the load16 with the use of the transistor 11 as part of a current source.Specifically, the nodeA is set at Vsig though it can be any potential,the nodeD is set at Vcat, the nodeG is set at VDD, and the nodeH can beset at Vinit though it can be any potential. Then, the switch 14 and theswitch 15 are turned on, and the switch 12, the switch 12 p, and theswitch 13 are turned off. Then, the nodeB and the nodeC become Vel, thenodeE becomes (Vsig+Vth−Vinit+Vx+Vel), and the nodeF becomes VDD. Inaddition, Vgs becomes (Vsig+Vth−Vinit+Vx) and Vc becomes(Vsig+Vth−Vinit+Vx).

The third operation described with reference to FIG. 17C is differentfrom that described in Embodiment 1 with reference to FIG. 3C in thatthe switch 12 p is turned off. Therefore, the third operation in thisembodiment is the same as the third operation described with referenceto FIG. 3C. By the third operation, Vgs of the transistor 11 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 11. Accordingly, through the above operations, adverse effectof variations in the threshold voltage of the transistor on the amountof current supplied to the load can be reduced.

Note that FIG. 15 illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The location of the switch or the number of switches can bechanged and appropriate voltage can be supplied so as to achieve thesimilar operation to the operation described in FIGS. 17A to 17C inwhich the threshold voltage of the transistor is corrected. In such amanner, a variety of circuits can be employed.

For example, specifically, the switch 12, the switch 12 p, the switch13, the switch 14, and the switch 15 can be provided any place and thenumbers thereof is not limited as long as the switches can control aconduction state and a non-conduction state between nodes. In the caseof the first operation described with reference to FIG. 17A, aconnection relation illustrated in FIG. 18A can be employed. In the caseof the second operation described with reference to FIG. 17B, aconnection relation illustrated in FIG. 18B can be employed. In the caseof the third operation described with reference to FIG. 17C, aconnection relation illustrated in FIG. 18C can be employed. Thepotential of each node can have any level unless the node affects theoperations.

As described above, in the circuit configuration described in thisembodiment, the wiring 18 p is provided and the initialization can beperformed with the use of Vinit supplied from the wiring 18 p throughthe switch 12 p. Accordingly, time for initializing the potential ofeach node can be long. Alternatively, the initialization with the use ofVinit is not necessarily performed by using the wiring 18, which cansave time allowing time for the second operation to be longer. In thethird operation, Vgs of the transistor 11 becomes (Vsig+Vth−Vinit+Vx),which includes the threshold voltage of the transistor 11. Accordingly,with this structure, adverse effect of variations in the thresholdvoltage of the transistor on the amount of current supplied to the loadcan be reduced.

Note that the operation for correcting mobility can be performed withthe use of the circuits illustrated in FIG. 15 and FIG. 16, as theoperations illustrated in FIG. 5C, FIG. 6C, FIG. 13A, and FIG. 13B.

Note that FIG. 15, or the like illustrates an example of a circuitconfiguration; accordingly, a transistor can be provided additionally.In each node in FIG. 15, or the like, it is possible not to provide anadditional transistor, switch, a passive element, or the like. Forexample, transistors directly connected to the nodeA, the nodeB, thenodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG are notadditionally provided. Accordingly, for example, the following structurecan be used: only the transistor 14T is directly connected to the nodeCand the other transistors are not directly connected to the nodeC.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 4

The operations of the circuit configurations are described inEmbodiments 1 to 3 under the assumption that the parasitic capacitanceof the load 16 is utilized; however, another configuration can be used.In this embodiment, a configuration in which a capacitor is electricallyconnected in parallel to the load 16 provided in the circuitconfiguration in any of the above embodiments.

FIG. 19 illustrates a semiconductor device 10 c and is different fromFIG. 1A in that a capacitor 17 c is electrically connected in parallelto the load 16 connected to the semiconductor device 10 c. The capacitor17 c can be connected to the wiring 20 as illustrated in FIG. 78.Alternatively, the capacitor 17 c can be connected to the wiring 32, thewiring 33, the wiring 34, the wiring 31 p, the wiring 18 p, or the like.Further alternatively, the capacitor 17 c can be connected to the wiring32, the wiring 33, the wiring 34, the wiring 31 p, or the like ofanother semiconductor device 10 c. Further, FIG. 20 illustrates asemiconductor device 10 hc and is different from FIG. 14 in that thecapacitor 17 c is electrically connected in parallel to the load 16connected to the semiconductor device 10 hc. The capacitor 17 c can beconnected to the wiring 20A or the wiring 20B as illustrated in FIG. 79or FIG. 80. Moreover, FIG. 21 illustrates a semiconductor device 10 pcand is different from FIG. 15 in that a capacitor 17 c is electricallyconnected in parallel to the load 16 connected to the semiconductordevice 10 hc. The capacitor 17 c can be connected to the wiring 18 p asillustrated in FIG. 81.

The capacitor 17 c is electrically connected to the load 16 asillustrated in FIG. 19, FIG. 20, and FIG. 21, so that variations inelectric charge at the nodeC can be small or Vx can be low in theoperation for initialization and the operation for obtaining thresholdvoltage which are described in any of the above embodiments. When Vx canbe low, the semiconductor device can supply a more accurate amount ofcurrent to the load 16.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 5

In this embodiment, a configuration different from the circuitconfigurations of the semiconductor devices described in Embodiments 1to 4 is described.

FIG. 22 illustrates a semiconductor device 10 hm having a circuitconfiguration similar to the semiconductor device 10 illustrated in FIG.1A. The semiconductor device 10 hm illustrated in FIG. 22 is differentfrom the semiconductor device 10 illustrated in FIG. 1A in that thesemiconductor device 10 hm is connected to a circuit 22 m. The circuit22 m which has a function of supplying a potential to the wiring 19 hasa function of supplying Vup or Vcat, switching them as necessary. Notethat components in common with those in FIG. 1A are denoted by commonreference numerals, and the description thereof is omitted.

The potential Vup can be higher than Vcat. The potential Vup is high, sothat Vinit is prevented from being too low.

Next, the operation of the semiconductor device 10 hm illustrated inFIG. 22 is described. The operation of the semiconductor device 10 hmillustrated in FIG. 22 can be mainly divided into a first operation, asecond operation, and a third operation.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 22, FIG. 22 shows symbols representing thepotentials of nodes between elements and the potentials of wirings, asFIG. 2B does. The operation of the circuit configuration illustrated inFIGS. 23A to 23C is explained with symbols of Vgs and Vc, as in FIG. 2B.

The first operation illustrated in FIG. 23A is the same as the firstoperation described with reference to FIG. 3A except that the nodeD isset at Vup. The description of the same portions is omitted. When thenodeD is set at Vup, current flowing to the load 16 in the firstoperation can be reduced more surely. Alternatively, a normal operationcan be performed with ease without making Vinit extremely low.Therefore, another potential can have smaller amplitude, resulting inreduction in power consumption.

The potential Vup is higher than Vinit and Vsig. Alternatively, Vup isapproximately equal to Vinit. Note that the potential is preferably setso as not to cause dielectric breakdown of the load 16.

The second operation illustrated in FIG. 23B is the same as the secondoperation described with reference to FIG. 3B except that the nodeD isset at Vup. The description of the same portions is omitted. When thenodeD is set at Vup, current flowing to the load 16 in the secondoperation can be reduced more surely.

Then, the third operation illustrated in FIG. 23C is the same as thethird operation described with reference to FIG. 31C and therefore thedescription thereof is omitted. Note that the third operationillustrated in FIG. 23C is different from the first operation describedwith reference to FIG. 23A and the second operation described withreference to FIG. 23B in that the nodeD is set at Vcat and current flowsthrough the load.

With the structure described with reference to FIGS. 23A to 23C, onlywhen the transistor 11 is completely set to allow the semiconductordevice to serve as a current source, current can flow without causingmalfunction.

Note that the operation for correcting mobility can be performed withthe use of the circuits illustrated in FIG. 22, as the circuitsillustrated in FIG. 5C, FIG. 6C, FIG. 13A, and FIG. 13B.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 6

In this embodiment, the structure used for part of a signal line drivercircuit of a display device including the semiconductor device describedin any of the above embodiments is explained.

As illustrated in FIG. 24, a display device 41 to which thesemiconductor device described in any of the above embodiments isapplied includes a pixel region 42, a gate line driver circuit 43, and asignal line driver circuit 44. The gate line driver circuit 43sequentially outputs a select signal to the pixel region 42. The signalline driver circuit 44 sequentially outputs a video signal to the pixelregion 42. The pixel region 42 displays an image by controlling thestate of light in accordance with a video signal. The video signal inputfrom the signal line driver circuit 44 to the pixel region 42 is acurrent. That is, a display element and an element for controlling thedisplay element arranged in each pixel change their states according tothe video signal (current) input from the signal line driver circuit 44.Examples of the display elements arranged in pixels are an EL element,an element used in an FED (Field Emission Display), a liquid crystalelement, electronic ink, an electrophoresis element, a grating lightvalve (GLV), and the like. Display devices having liquid crystalelements include a liquid crystal display (e.g., a transmissive liquidcrystal display, a transflective liquid crystal display, a reflectiveliquid crystal display, a direct-view liquid crystal display, or aprojection liquid crystal display) and the like. Display devices havingelectronic ink or electrophoretic elements include electronic paper andthe like.

Note that the number of the gate line driver circuits 43 and the signalline driver circuits 44 may be more than one.

The signal line driver circuit 44 can be divided into a plurality ofportions in its configuration. As an example, it can be roughly dividedinto a shift register 45, a first latch circuit 46 (LAT1), a secondlatch circuit 47 (LAT2), and a digital to analog converter circuit 48.The digital to analog converter circuit 48 has a function of convertinga voltage into a current, and it may also have a function of providing agamma correction. That is, the digital to analog converter circuit 48has a circuit which outputs a current (video signal) to a pixel, namelya current source circuit to which the semiconductor device described inany of the above embodiments can be applied.

In addition, a pixel has a display element such as an EL element. Thepixel has a circuit which outputs a current (video signal) to thedisplay element, namely a current source circuit to which thesemiconductor device described in any of the above embodiments can alsobe applied.

Next, the operation of the signal line driver circuit 44 is describedbriefly. The shift register 45 is formed by using a plurality of columnsof flip-flop circuits (FFs) and the like, and a clock signal (S-CLK), astart pulse (SP), and an inverted clock signal (S-CLKb) are input to theshift register 45. Sampling pulses are sequentially outputted inaccordance with these signals.

The sampling pulse outputted from the shift register 45 is input to thefirst latch circuit 46 (LAT1). The first latch circuit 46 (LAT1) isinput with a video signal (VS) from the video signal line and holds avideo signal in each column in response to the timing at which thesampling pulses are input. Note that a video signal has a digital valuein the case where the digital to analog converter circuit 48 isdisposed. Further, a video signal in this stage is often a voltage.

However, in a case where the first latch circuit 46 and the second latchcircuit 47 are circuits which can store analog values, the digital toanalog converter circuit 48 can be omitted in many cases. In that case,a video signal is a current in many cases. Further, in a case where dataoutput to the pixel region 42 has a binary value, that is a digitalvalue, the digital to analog converter circuit 48 can be omitted in manycases.

When the retainment of the video signals up to the last column iscompleted in the first latch circuit 46 (LAT1), a latch pulse LP isinput from a latch control line in a horizontal retrace period and thevideo signals held in the first latch circuit 46 (LAT1) are transferredto the second latch circuit 47 (LAT2) all at once. After that, the videosignals of one row, which are held in the second latch circuit 47(LAT2), are input to the digital to analog converter circuit 48 at once.Then, a signal output from the digital to analog converter circuit 48 isinput to the pixel region 42.

While the video signal held in the second latch circuit 47 (LAT2) isinput to the digital to analog converter circuit 48 and then inputted tothe pixel region 42, a sampling pulse is outputted from the shiftregister 45 again. In other words, two operations are performed at thesame time. Accordingly, a line sequential driving can be enabled. Theseoperations are repeated thereafter.

When a current source circuit in the digital to analog converter circuit48 is a circuit which performs the set operation and the outputoperation, a circuit to supply a current to the current source circuitis required. In that case, a reference current source circuit 49 isdisposed.

Note that a part or all of the signal line driver circuit may beprovided outside a substrate having the pixel region 42, and forexample, it may be constructed of an external IC chip. In that case, theIC chip and the substrate are connected by using COG (Chip On Glass),TAB (Tape Auto Bonding), a printed substrate or the like.

Note that a configuration of the signal line driver circuit or the likeis not limited to FIG. 24.

For example, in a case where the first latch circuit 46 and the secondlatch circuit 47 can store analog values, a video signal VS (analogcurrent) is input to the first latch circuit 46 (LAT1) from a referencecurrent source circuit 50 as illustrated in FIG. 25 in some cases.Further, the second latch circuit 47 is not included in FIG. 25 in somecases.

Next, a specific configuration where the semiconductor device describedin any of the above embodiments is applied to the signal line drivercircuit 44 is described.

First, FIG. 26 illustrates an example of a circuit configuration of thesemiconductor device described in any of the above embodiments which isapplied to the signal line driver circuit. The semiconductor device 10_1illustrated in FIG. 26 has the configuration similar to thesemiconductor device 10 described with reference to FIG. 1A inEmbodiment 1. Note that components in common with those in FIG. 1A aredenoted by common reference numerals, and the description thereof isomitted. Current in accordance with Vsig of the circuit 21 can be outputto the load because variations in the threshold voltage of thetransistor 11 can be reduced.

Supply of current in accordance with Vsig set in the semiconductordevice 10_1 is controlled by the switching of a switch 60_1 providedbetween the semiconductor device 10_1 and the load 16. In that case, forexample, the plurality of semiconductor devices 10_1 are provided andthe amount of current flowing to the load can be controlled by theswitches 60_1.

For example, the following structure illustrated in FIG. 27 can be alsoused: the semiconductor devices 10_1 to 10_3 are provided as theplurality of semiconductor devices and the amount of current flowing tothe load 16 is changed by control of the switches 60_1 to 60_3. Theamounts of currents flowing at the semiconductor devices 10_1 to 10_3are set in the circuit 21 so as to differ from each other or be the sameas each other, and the amount of current flowing to the load 16 may becontrolled by the switching of the switches.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 7

In this embodiment, an example of the case where the circuitconfiguration which is one embodiment of the present invention isapplied to a pixel of a display device is described.

FIG. 28A illustrates a circuit configuration of a pixel. Note that inthis embodiment, an n-channel transistor is described as an example. Inthe structure described below, a light-emitting element is used as adisplay element included in the pixel.

The circuit configuration illustrated in FIG. 28A has a circuit fordischarging electric charge held in a gate of a transistor in order tocorrect variations in current characteristics due to variations inthreshold voltage of the transistor or the like. In practice, the pixelcircuit has a connection relation so that variations in currentcharacteristics of the transistor can be corrected by controlling theswitching of a plurality of switches provided between wirings.

The pixel 100 illustrated in FIG. 28A includes a switch 102, a switch103, a switch 104, a switch 105, a light-emitting element 106, acapacitor 107, and a transistor 101 which allows the pixel 100 tooperate as a current source. Note that in this embodiment, thetransistor 101 which allows the semiconductor device to operate as acurrent source is an n-channel transistor, for example.

Next, a connection relation of components in the pixel 100 is described.

A gate of the transistor 101 is connected to one of electrodes of thecapacitor 107 and one of terminals of the switch 103. A first terminal(one of a source and a drain) of the transistor 101 is connected to oneof terminals of the switch 102 and one of terminals of the switch 104. Asecond terminal (the other of the source and the drain) of thetransistor 101 is connected to the other of the terminals of the switch103 and one of terminals of the switch 105. Note that one of terminalsis also referred to as a first terminal and the other of the terminalsis also referred to as a second terminal.

The other of the terminals of the switch 102 is connected to the wiring108.

The other of the terminals of the switch 105 is connected to the wiring110.

One of electrodes of the light-emitting element 106 is connected to theother of the terminals of the switch 104 and the other of the electrodesof the capacitor 107. The other of the electrodes of the light-emittingelement 106 is connected to the wiring 109.

Note that in this embodiment, a pixel corresponds to a display unitcontrolling the luminance of one color component (e.g., any one of R(red), G (green), and B (blue)). Therefore, in a color display device,the minimum display unit of a color image is composed of three pixels ofan R pixel, a G pixel and a B pixel. Note that the color of the colorelements is not necessarily of three varieties and may be of three ormore varieties or may include a color other than RGB.

An example of the light-emitting element is an EL element. Examples ofan EL element are an element including an anode, a cathode, and an ELlayer interposed between the anode and the cathode, and the like.Examples of an EL layer are a layer utilizing light emission(fluorescence) from a singlet exciton, a layer utilizing light emission(phosphorescence) from a triplet exciton, a layer utilizing lightemission (fluorescence) from a singlet exciton and light emission(phosphorescence) from a triplet exciton, a layer formed using anorganic material, a layer formed using an inorganic material, a layerformed using an organic material and an inorganic material, a layerincluding a high-molecular material, a layer including a low-molecularmaterial, a layer including a high-molecular material and alow-molecular material, and the like. Note that the present invention isnot limited thereto, and various types of EL elements can be used.

Note that as illustrated in FIG. 28B, the wiring 108 is connected to acircuit 121 having at least a function of supplying Vinit or Vsig byswitching Vinit and Vsig. An example of the circuit 121 is a sourcedriver (signal line driver circuit). Accordingly, the wiring 108 has afunction of capable of transmitting or supplying Vinit and/or Vsig.

The potential Vinit initializes the potential of each node in the pixelbefore Vsig which is a video signal is supplied, for example. Note thatVinit may be different depending on pixels, rows, or columns.Alternatively, Vinit may be different depending on colors of pixels.

An example of Vsig is a video signal. Therefore, the potential to besupplied to pixels depends on an image to be displayed. When the imageto be displayed is a moving image, the potential to be supplied variesover time in some cases. Further, when the image to be displayed is astill image, the fixed potential is supplied in some cases.

Note that as illustrated in FIG. 28B, the wiring 109 is connected to acircuit 122 having at least a function of supplying Vcat, for example.An example of the circuit 122 is a power supply circuit. Accordingly,the wiring 109 has a function of capable of transmitting or supplyingVcat.

The potential Vcat is set to make current flow from the side of one ofelectrodes (an anode) of the light-emitting element 106 to the side ofthe other of the electrodes (cathode) of the light-emitting element 106in a period in which the light-emitting element 106 emits light. If thecathodes of the light-emitting elements 106 in pixels are the same, thewiring of each pixel is supplied with Vcat. The potential Vcat may bedifferent depending on pixels, rows, or columns. Alternatively, Vcat maybe different depending on colors of pixels.

Note that as illustrated in FIG. 28B, the wiring 110 is connected to atleast a circuit 123 for supplying VDD. An example of the circuit 123 isa power supply circuit. Accordingly, the wiring 110 has a function ofcapable of transmitting or supplying VDD. Alternatively, the wiring 110has a function of capable of supplying current to the transistor 101.Alternatively, the wiring 110 has a function of capable of supplyingcurrent to the light-emitting element 106.

The potential VDD is set to make current flow from the side of the oneof the electrodes of the light-emitting element 106 to the side of theother of the electrodes of the light-emitting element 106 through thetransistor 101. Therefore, for example, VDD is higher than Vcat. Whenthe characteristics of the light-emitting elements 106 in pixels are thesame, the same VDD can be supplied to the wiring of each pixel. Thepotential VDD may be different depending on pixels, rows, or columns.Alternatively, VDD may be different depending on colors of pixels.

Note that each of the switch 102, the switch 103, the switch 104, andthe switch 105 which are illustrated in FIG. 28A can be a transistor.Thus, as an example, FIG. 29A illustrates the case where an n-channeltransistor is used as each of the switch 102, the switch 103, the switch104, and the switch 105. Note that components in common with those inFIG. 28A are denoted by common reference numerals, and the descriptionthereof is omitted. All of the transistors have the same polarity asillustrated in FIG. 29A, whereby the semiconductor device can bemanufactured in a small number of steps. Thus, the manufacturing costcan be reduced.

In FIG. 29A, a transistor 102T corresponds to the switch 102, atransistor 103T corresponds to the switch 103, a transistor 104Tcorresponds to the switch 104, and a transistor 105T corresponds to theswitch 105.

A gate of the transistor 102T is connected to a wiring 131. A firstterminal of the transistor 102T is connected to the first terminal ofthe transistor 101 and a first terminal of the transistor 104T. A secondterminal of the transistor 102T is connected to the wiring 108.Therefore, the transistor 102T is in a conduction state when thepotential of the wiring 131 is at an H level, and the transistor 102T isin a non-conduction state when the potential of the wiring 131 is at anL level.

Further, a gate of the transistor 103T is connected to a wiring 132. Afirst terminal of the transistor 103T is connected to the gate of thetransistor 101 and the one of the electrodes of the capacitor 107. Asecond terminal of the transistor 103T is connected to a first terminalof the transistor 105T and the second terminal of the transistor 101.Therefore, the transistor 103T is in a conduction state when thepotential of the wiring 132 is at an H level, and the transistor 103T isin a non-conduction state when the potential of the wiring 132 is at anL level.

Furthermore, a gate of the transistor 104T is connected to a wiring 133.A first terminal of the transistor 104T is connected to the firstterminal of the transistor 101 and the first terminal of the transistor102T. A second terminal of the transistor 104T is connected to the firstelectrode of the light-emitting element 106 and the other electrode ofthe capacitor 107. Therefore, the transistor 104T is in a conductionstate when the potential of the wiring 133 is at an H level, and thetransistor 104T is in a non-conduction state when the potential of thewiring 133 is at an L level.

In addition, a gate of the transistor 105T is connected to a wiring 134.The first terminal of the transistor 105T is connected to the secondterminal of the transistor 101 and the second terminal of the transistor103T. A second terminal of the transistor 105T is connected to thewiring 110. Therefore, the transistor 105T is in a conduction state whenthe potential of the wiring 134 is at an H level, and the transistor105T is in a non-conduction state when the potential of the wiring 134is at an L level.

Note that the wiring 131, the wiring 132, the wiring 133, and the wiring134 can function as different wirings. However, one embodiment of thepresent invention is not limited thereto. The wirings 131 to 134 can becombined into one wiring; therefore, it is possible to form a circuitwith a small number of wirings.

For example, the wiring 131 and the wiring 132 can be combined into onewiring. Therefore, the wiring 131 can be connected to the wiring 132 tobe one wiring. At this time, the transistor 102T and the transistor 103Tpreferably have the same polarity. FIG. 96 shows a circuit diagram ofthis case.

For example, the wiring 133 and the wiring 134 can be combined into onewiring. Therefore, the wiring 133 can be connected to the wiring 134 tobe one wiring. At this time, the transistor 104T and the transistor 105Tpreferably have the same polarity. FIG. 97 shows a circuit diagram ofthis case.

Note that the wiring 131 and the wiring 132 can be combined into onewiring and the wiring 133 and the wiring 134 can be combined into onewiring. FIG. 98 shows a circuit diagram in that case.

In many cases, the transistor 101 operates in a saturation region at thetime of passing current. Therefore, the transistor 101 preferably has alonger channel length or gate length than the transistor 102T, thetransistor 103T, the transistor 104T, or the transistor 105T. When thechannel length or the gate length is longer, characteristics in asaturation region have a flat slope; accordingly, a kink effect can bereduced. Note that one embodiment of the present invention is notlimited to these examples.

In many cases, the transistor 101 operates in a saturation region at thetime of passing current. Therefore, the transistor 101 preferably has alarger channel width or gate width than the transistor 102T, thetransistor 103T, the transistor 104T, or the transistor 105T. When thechannel width or the gate width is larger, a large amount of current canflow even in a saturation region. Note that one embodiment of thepresent invention is not limited to these examples.

Here, a display device including the pixel 100 is described withreference to a block diagram of FIG. 30.

The display device includes a signal line driver circuit 201, a scanline driver circuit 202A, a scan line driver circuit 202B, a scan linedriver circuit 202C, a scan line driver circuit 202D, and a pixel region203. The pixel region 203 is provided with a plurality of signal linesS1 to Sn (n is a natural number) extended from the signal line drivercircuit 201 in a column direction; a plurality of scan lines Ga1 to Gam(m is a natural number) extended from the scan line driver circuit 202Ain a row direction; a plurality of scan lines Gb1 to Gbm extended fromthe scan line driver circuit 202B in a row direction; a plurality ofscan lines Gc1 to Gcm extended from the scan line driver circuit 202C ina row direction; a plurality of scan lines Gd1 to Gdm extended from thescan line driver circuit 202D in a row direction; a plurality of pixels100 provided in matrix, connected to respective signal lines S1 to Sn,and connected to respective scan lines Ga1 to Gam, Gb1 to Gbm, Gc1 toGcm, and Gd1 to Gdm; and power supply lines P1 to Pn which are parallelto the signal lines S1 to Sn. The pixel 100 is connected to the signalline Sj (one of the signal lines S1 to Sn), the scan line Gai (one ofthe scan lines Ga1 to Gam), the scan line Gbi (one of the scan lines Gbito Gbm), the scan line Gci (one of the scan lines Gc1 to Gcm), the scanline Gdi (one of the scan lines Gd1 to Gdm), and the power supply linePj (one of the power supply lines P1 to Pn). Note that i and j arenatural numbers.

The scan line Gai corresponds to the wiring 131 in FIG. 29A. The scanline Gbj corresponds to the wiring 132 in FIG. 29A. The scan line Gcjcorresponds to the wiring 133 in FIG. 29A. The scan line Gdj correspondsto the wiring 134 in FIG. 29A. The signal line Sj corresponds to thewiring 108 in FIG. 29A. The power supply line Pj corresponds to thewiring 110 in FIG. 29A. Although not illustrated in FIG. 30, a cathodeline which the pixels use in common and the cathode line corresponds tothe wiring 109.

A scan line is selected with the use of a signal output from the scanline driver circuits 202A to 202D. The potential of each node of thepixels 100 connected to the selected scan line is initialized (firstoperation). Then, a video signal is written to the initialized pixel 100to obtain the threshold voltage of a transistor (second operation).After the threshold voltage of the transistor is obtained by writing ofthe video signal, the operation moves to light emission. The pixel emitslight in accordance with the video signal written to the pixel (thirdoperation). In this manner, the initialization of the pixel 100,obtaining of the threshold voltage, and light-emitting operation aresequentially performed.

Next, the operation of the pixel 100 illustrated in FIG. 28A isdescribed. The operation of the pixel 100 illustrated in FIG. 28A can bemainly divided into a first operation, a second operation, and a thirdoperation. Note that one embodiment of the present invention is notlimited thereto, and another operation can be added or part of theoperation can be skipped.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 28A, FIG. 29B shows symbols representing thepotentials of nodes between elements and the potentials of wirings. FIG.29B also shows Vgs between the one of the terminals (mainly serving as asource) and the gate of the transistor 101 and Vc between the electrodesof the capacitor 107.

A nodeA, a nodeB, a nodeC, a nodeD, a nodeE, a nodeF, and a nodeGcorrespond to nodes and wirings illustrated in FIG. 29B. The potentialof the nodeA corresponds to the potential of the wiring 108. Thepotential of the nodeB corresponds to the potential of a wiringconnecting the first terminal of the transistor 101, the first terminalof the switch 102, and the first terminal of the switch 104. Thepotential of the nodeC corresponds to the potential of a wiringconnecting the second terminal of the switch 104, the one of theelectrodes of the light-emitting element 106, and the other of theelectrodes of the capacitor 107. The potential of the nodeD correspondsto the potential of the wiring 109. The potential of the nodeEcorresponds to the potential of a wiring connecting the gate of thetransistor 101, the one of electrodes of the capacitor 107, and thefirst terminal of the switch 103. The potential of the nodeF correspondsto the potential of a wiring connecting the second terminal of thetransistor 101, the second terminal of the switch 103, and the firstterminal of the switch 105. The potential of the nodeG corresponds tothe potential of the wiring 110.

First, the first operation is described with reference to FIG. 31A. Notethat reference numerals of elements in FIG. 31A are omitted. Note thatin the drawings, a conduction state and a non-conduction state of theswitches are denoted by ON and OFF. In addition, how Vgs, Vc, thepotential of the nodeA, the potential of the nodeB, the potential of thenodeC, the potential of the nodeD, the potential of the nodeE, thepotential of the nodeF, and the potential of the nodeG, which areillustrated in FIG. 29B, are applied is described.

The first operation initializes the potential of each node in the pixel100. Specifically, the nodeA is set at Vinit, the nodeD is set at Vcat,and the nodeG is set at VDD. Then, the switch 102, the switch 103, theswitch 104, and the switch 105 are turned on. Thus, the nodeB is set atVinit, the nodeC is set at Vinit, the nodeE is set at VDD, and the nodeFis set at VDD. Further, Vgs becomes (VDD−Vinit), and Vc becomes(VDD−Vinit).

As described above, in the first operation, Vinit at the nodeB and thenodeC is equal to or lower than Vcat at the nodeD, for example. Withthis structure, current is prevented from flowing to the light-emittingelement 106 in the first operation. Accordingly, problems caused bycurrent flowing to the light-emitting element 106 can be reduced.Further, when Vinit is lower than Vcat, the light-emitting element 106can be reverse biased. In that case, deterioration of the light-emittingelement 106 can be reduced and the light-emitting element 106 can berepaired.

In the first operation, VDD at the nodeE and the nodeF is higher thanVcat at the nodeD. With this structure, Vgs can be higher than thethreshold voltage of the transistor 101 in the first operation.Alternatively, electric charge can be charged in the capacitor 107.

Next, the second operation is described with reference to FIG. 31B, asin FIG. 31A.

The second operation is the operation for obtaining the thresholdvoltage of the transistor 101 as Vgs by discharging the potential of thegate of the transistor 101 (or the electric charge charged in thecapacitor 107). Specifically, the nodeA is set at Vsig, the nodeD is setat Vcat, and the nodeG is set at VDD. Then, the switch 102 and theswitch 103 are turned on, and the switch 104 and the switch 105 areturned off. Accordingly, the potential of the nodeB becomes Vsig, thepotential of the nodeC becomes (Vinit−Vx), the potential of the nodeEbecomes (Vsig+Vth), and the potential of the nodeF becomes (Vsig+Vth).Further, Vgs becomes Vth and Vc becomes (Vsig+Vth−Vinit+Vx).

As described above, Vsig at the nodeB in the second operation is thepotential used for controlling the amount of current flowing between thewiring 110 and the wiring 109 with the use of the transistor 101 in thethird operation. By the second operation, the potential of the nodeEcorresponding to the potential of the gate of the transistor 101 can be(Vsig+Vth) which includes the threshold voltage of the transistor 101.

In the second operation, Vx of the potential of the nodeC (Vinit−Vx)changes when the nodeC is set in an electrically floating state. In thiscase, the amount of changes in Vx depends on a ratio of capacitance ofthe light-emitting element 106 to the capacitance of the capacitor 107.Note that Vx is preferably set to a low potential in advance.Specifically, the capacitance of the light-emitting element 106 is setto be sufficiently larger than the capacitance of the capacitor 107,whereby Vx can be low. It is preferable that the capacitance of thelight-emitting element 106 be two times or more, more preferably fourtimes or more the capacitance of the capacitor 107.

Further, VDD at the nodeD and the nodeE in the first operation isdischarged by the second operation. By the discharging, Vgs is decreasedto the threshold voltage Vth of the transistor 101 and is set in asteady state. Therefore, the discharging makes the nodeD and the nodeEare set in a steady state at (Vsig+Vth). In addition, at the time ofterminating the second operation, (Vsig+Vth−Vinit+Vx) is held as Vc.

Note that in some cases, it takes a very long time until Vgs becomesequal to the threshold voltage Vth of the transistor 101. Accordingly,in many case, the semiconductor device is driven while Vgs is notcompletely decreased to the threshold voltage Vth. That is, in manycases, the second operation is terminated while Vgs is slightly higherthan the threshold voltage Vth. In other words, at the time ofterminating the second operation, Vgs is based on the threshold voltage.

Note that in the second operation, the switch 104 and the switch 105 areturned off and the potential of the nodeB is set to Vsig. Theseoperations can be performed at the same time or at different timings.

It is preferable that, for example, the potential of the nodeB bechanged from Vinit to Vsig at the same time as or after the switch 104is turned off. This is because the potential of the nodeC can be easilyheld at an appropriate potential.

Alternatively, it is preferable that, for example, the potential of thenodeB be changed from Vinit to Vsig before or at the same time as theswitch 105 is turned off. This is because the gate potential of thetransistor can be quickly lowered.

Next, the third operation is described with reference to FIG. 31C, as inFIGS. 31A and 31B.

The third operation is the operation for outputting current to thelight-emitting element 106 and emitting light with the use of thetransistor 101 as part of a current source. Specifically, the nodeA isset at Vsig, for example, though it can be any potential, the nodeD isset at Vcat, and the nodeG is set at VDD. Then, the switch 104 and theswitch 105 are turned on, and the switch 102 and the switch 103 areturned off. Accordingly, the nodeB and the nodeC become Vel, the nodeEbecomes (Vsig+Vth−Vinit+Vx+Vel), and the nodeF becomes VDD. In addition,Vgs becomes (Vsig+Vth−Vinit+Vx) and Vc becomes (Vsig+Vth−Vinit+Vx).

Note that in the third operation, the potentials of the nodeB, thenodeC, and the nodeF are increased while the nodeE is kept in anelectrically floating state. Accordingly, the potential of the nodeE isincreased by capacitive coupling while (Vsig+Vth−Vinit+Vx) is held asVc, thereby becoming (Vsig+Vth−Vinit+Vx+Vel). That is, an increase inthe potential of the nodeC leads to an increase in the potential of thenodeE by bootstrap operation.

The semiconductor device can operate even when the potential of thenodeC is increased; therefore, adverse effect of deterioration involtage current characteristics of the light-emitting element 106 can bereduced even when the deterioration is caused.

The potential Vel which is the potentials of the nodeB and the nodeC isset when the potential of the nodeF is increased to VDD and currentflows to the light-emitting element 106 through the transistor 101 whichallows the semiconductor device to operate as a current source by thethird operation. Specifically, the potential ranges from VDD to Vcat.

In the third operation, Vgs of the transistor 101 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 101. The amount of current of the transistor 101 depends on(Vgs−Vth). Accordingly, through the above operations, adverse effect ofvariations in the threshold voltage of the transistor on the amount ofcurrent supplied to the light-emitting element can be reduced.Alternatively, even when the threshold voltage is changed bydeterioration of the transistor, adverse effect of the change can bereduced. Therefore, display unevenness can be reduced and display can beperformed with high quality.

Note that in the third operation, the switch 102 and the switch 103 areturned off and the switch 104 and the switch 105 are turned on. Theseoperations can be performed at the same time or at different timings.

For example, it is preferable that the switch 104 and the switch 105 beturned on after the switch 102 and the switch 103 are turned off. Thisis because Vc can be easily held at an appropriate potential.

Alternatively, for example, it is preferable that the switch 102 beturned off after the switch 103 is turned off. This is because Vc can beeasily held at an appropriate potential.

Note that FIG. 28A illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The locations of the switches or the number of switches can bechanged and/or appropriate voltage can be supplied so that theoperations become similar to the operations described in FIGS. 31A to31C in which the threshold voltage of the transistor is corrected. Insuch a manner, a variety of circuits can be employed.

For example, specifically, the switch 102, the switch 103, the switch104, and the switch 105 can be provided at any place and the number ofswitches is not limited as long as the switches can control a conductionstate and a non-conduction state between nodes. In the case of the firstoperation described with reference to FIG. 31A, a connection relationillustrated in FIG. 32A may be employed. In the case of the secondoperation described with reference to FIG. 31B, a connection relationillustrated in FIG. 32B can be employed. In the case of the thirdoperation described with reference to FIG. 31C, a connection relationillustrated in FIG. 32C can be employed. The potential of each node canhave any level unless the node affects the operations.

Note that the operation for correcting the threshold voltage of thetransistor is described with reference to FIGS. 31A to 31C but oneembodiment of the present invention is not limited thereto. For example,the operation for correcting variations in the mobility of thetransistor 101 may be performed between the second operation in FIG. 31Band the third operation in FIG. 31C. FIGS. 33A to 33D illustrate thecase where the operation for correcting variations in the mobility ofthe transistor 101 is added to the first to third operations which aredescribed with reference to FIGS. 31A to 31C.

Note that a first operation illustrated in FIG. 33A is the same as thefirst operation described with reference to FIG. 31A; therefore, thedescription thereof is omitted. A second operation illustrated in FIG.33B is the same as the second operation described with reference to FIG.31B; therefore, the description thereof is omitted.

Next, a third operation is described with reference to FIG. 33C, as inFIGS. 31A and 31B.

In the third operation, the transistor 101 is turned on with the use ofthe potential held in the gate of the transistor 101 (electric chargestored in the capacitor 107), and the mobility of the transistor 101 iscorrected with the use of the amount of current flowing therethrough.Specifically, the nodeA is set at Vsig though it can be any potential,the nodeD is set at Vcat, and the nodeG is set at VDD. Then, the switch103 and the switch 104 are turned on, and the switch 102 and the switch105 are turned off. Then, the amount of change in potentials of thenodeB and the nodeC is −ΔVel, the nodeE and the nodeF become(Vsig+Vth−ΔVel). In addition, Vgs becomes (Vth+ΔVel) and Vc becomes(Vsig+Vth−Vinit+Vx−ΔVel).

Note that in the third operation, the potentials of the nodeB and thenodeC are changed by turning on the switch 104. The amount of changes inthe potentials corresponds to −ΔVel. When the amount of change in thepotentials of the nodeB and the nodeC becomes −ΔVel, Vgs becomes(Vth+ΔVel) and higher than the threshold voltage Vth; as a result,current flows through the transistor 101. When current flows through thetransistor 101, each of the potentials of the nodeE and the nodeF isdecreased to (Vsig+Vth−ΔVel) and Vc becomes (Vsig+Vth−Vinit+Vx−ΔVel).

The amount of current flowing to the transistor 101 changes depending onthe mobility of the transistor 101. Accordingly, the potential of thenodeE corresponding to the gate of the transistor 101 can be set so asto include the amount of change in potential corresponding to themobility of the transistor 101.

In the third operation, the potential of the gate of the transistor 101becomes (Vsig+Vth−ΔVel) which is set in consideration of the mobility ofthe transistor 101. Accordingly, through the above operations, adverseeffect of variations in the mobility of the transistor on the amount ofcurrent supplied to the light-emitting element can be reduced.Alternatively, even when mobility is changed by deterioration of thetransistor, adverse effect of the change can be reduced.

Next, a fourth operation is described with reference to FIG. 33D, as inFIGS. 31A and 31B. Note that the fourth operation illustrated in FIG.33D is similar to the third operation described with reference to FIG.31C; therefore, only an aspect different from the third operation withreference to FIG. 31C is described.

By the fourth operation, the nodeB and the nodeC are set at Vel, thenodeE is set at (Vsig+Vth−Vinit+Vx−ΔVel+Vel), and the nodeF is set atVDD. The potential Vgs becomes (Vsig+Vth−Vinit+Vx−ΔVel) and Vc becomes(Vsig+Vth−Vinit+Vx).

In the fourth operation, Vgs of the transistor 101 becomes(Vsig+Vth−Vinit+Vx+ΔVel), which can be set in consideration of thethreshold voltage and the mobility of the transistor 101. Accordingly,through the above operations, adverse effect of variations in thethreshold voltage and the mobility of the transistor on the amount ofcurrent supplied to the light-emitting element can be reduced.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIGS. 33A to 33D in which thethreshold voltage of the transistor is corrected. In such a manner, avariety of circuits can be employed.

For example, specifically, the switch 102, the switch 103, the switch104, and the switch 105 can be provided any place and the numbersthereof is not limited as long as the switches can control a conductionstate and a non-conduction state between nodes. In the case of the firstoperation described with reference to FIG. 33A, a connection relationillustrated in FIG. 34A can be employed. In the case of the secondoperation described with reference to FIG. 33B, a connection relationillustrated in FIG. 34B can be employed. In the case of the thirdoperation described with reference to FIG. 33C, a connection relationillustrated in FIG. 34C can be employed. In the case of the fourthoperation described with reference to FIG. 33D, a connection relationillustrated in FIG. 34D can be employed. The potential of each node canhave any level unless the node affects the operations.

Note that FIG. 28A illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The number of transistors 101 or the locations of thetransistors 101 can be changed and a variety of circuits can beemployed.

For example, as in a pixel 100A illustrated in FIG. 35, the transistor101A and the transistor 101B which have gates connected to each otherand which are connected in series can be used as transistors which allowthe semiconductor device to serve as a current source. Note thatcomponents in common with those in FIG. 28A are denoted by commonreference numerals, and the description thereof is omitted.

As another example, as in a pixel 100B illustrated in FIG. 36, thetransistor 101A and the transistor 101B which have gates connected toeach other and which are connected in parallel can be used astransistors which allow the semiconductor device to serve as a currentsource. Note that components in common with those in FIG. 28A aredenoted by common reference numerals, and the description thereof isomitted.

As another example, as in a pixel 100C illustrated in FIG. 37, thetransistor 101A, the transistor 101B, the transistor 101C, and thetransistor 101D which have gates connected to each other and which areconnected in series and parallel can be used as transistors which allowthe semiconductor device to serve as a current source. Note thatcomponents in common with those in FIG. 28A are denoted by commonreference numerals, and the description thereof is omitted.

The channel width and/or the channel length of the transistor 101 can bechanged by application of the structures illustrated in FIG. 35, FIG.36, and FIG. 37. With the structures illustrated in FIG. 35, FIG. 36,and FIG. 37 in which channel widths and/or channel lengths of aplurality of transistors can be changed after the transistors arecombined, adverse effect of variations in characteristics of thetransistors can be smaller in comparison with the structure in whichtransistors each having a large channel width and/or a large channellength is provided in advance.

Note that FIG. 28A, FIG. 29A, or the like illustrates an example of acircuit configuration; accordingly, a transistor can be providedadditionally. On the other hand, in each node in FIG. 28A, FIG. 29A, orthe like, it is also possible not to provide an additional transistor,switch, passive element, or the like. For example, it is possible not toincrease the number of transistors directly connected to the nodeA, thenodeB, the nodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG.Accordingly, for example, the following structure can be used: only thetransistor 104T is directly connected to the nodeC and the othertransistors are not directly connected to the nodeC.

Therefore, a circuit can be formed with a small number of transistors inthe case where a transistor is not added.

Note that variations in the threshold voltage or the like of atransistor is corrected in this embodiment, but one embodiment of thepresent invention is not limited thereto. For example, current can besupplied to the light-emitting element 106 and the semiconductor devicecan be driven without performing the operation for correcting variationsin threshold voltage.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 8

In this embodiment, an example of a configuration different from thecircuit configuration of the pixel described in Embodiment 7 isdescribed.

FIG. 38 illustrates a pixel 100 h having a circuit configuration similarto the pixel 100 illustrated in FIG. 30. The pixel 100 h illustrated inFIG. 38 is different from the pixel 100 illustrated in FIG. 30 in thatthe wiring 110 is replaced with a wiring 110 h provided in parallel tothe wirings 131 to 134, Vsig and Vinit are supplied from the wiring 108,and at least Vinit or VDD is supplied, switching them as necessary, fromthe wiring 110 h. Note that components in common with those in FIG. 30are denoted by common reference numerals, and the description thereof isomitted.

Here, a display device including the pixel 100 h is described withreference to a block diagram of FIG. 39.

The display device includes a signal line driver circuit 201, a scanline driver circuit 202A, a scan line driver circuit 202B, a scan linedriver circuit 202C, a scan line driver circuit 202D, a pixel region203, and a power supply line control circuit 204. The pixel region 203is provided with a plurality of signal lines S1 to Sn extended from thesignal line driver circuit 201 in a column direction; a plurality ofscan lines Ga1 to Gam extended from the scan line driver circuit 202A ina row direction; a plurality of scan lines Gb1 to Gbm extended from thescan line driver circuit 202B in a row direction; a plurality of scanlines Gc1 to Gcm extended from the scan line driver circuit 202C in arow direction; a plurality of scan lines Gd1 to Gdm extended from thescan line driver circuit 202D in a row direction; a plurality of pixels100 provided in matrix, connected to respective signal lines S1 to Sn,and connected to respective scan lines Ga1 to Gam, Gb1 to Gbm, Gc1 toGcm, and Gd1 to Gdm; and power supply lines P1 to Pm which are parallelto the scan lines Ga1 to Gam, Gb1 to Gbm, Gc1 to Gcm, and Gd1 to Gdm.The pixel 100 h is connected to the signal line Sj (one of the signallines S1 to Sn), the scan line Gai (one of the scan lines Ga1 to Gam),the scan line Gbi (one of the scan lines Gb1 to Gbm), the scan line Gci(one of the scan lines Gc1 to Gcm), the scan line Gdi (one of the scanlines Gd1 to Gdm), and the power supply line Pj (one of the power supplylines P1 to Pn).

The scan line Gai corresponds to the wiring 131 in FIG. 38. The scanline Gbj corresponds to the wiring 132 in FIG. 38. The scan line Gcjcorresponds to the wiring 133 in FIG. 38. The scan line Gdj correspondsto the wiring 134 in FIG. 38. The signal line Sj corresponds to thewiring 108 in FIG. 38. The power supply line Pj corresponds to thewiring 110 h in FIG. 38. Although not illustrated in FIG. 39, a cathodeline which the pixels use in common and the cathode line corresponds tothe wiring 109.

A scan line is selected with the use of a signal output from the scanline driver circuits 202A to 202D. An operation in which a potential forinitialization is applied to some extent to each node of the pixels 100connected to the selected scan line before initialization of thepotential of each node (initialization before initialization) isperformed (first operation). The potential of each node of the pixels100 connected to the selected scan line is initialized (secondoperation). Then, a video signal is written to the initialized pixel 100to obtain the threshold voltage of a transistor (third operation). Afterthe threshold voltage of the transistor is obtained by writing of thevideo signal, the operation moves to light emission. The pixel emitslight in accordance with the video signal written to the pixel (fourthoperation). In this manner, the initialization before theinitialization, the initialization of the pixel 100, obtaining of thethreshold voltage, and a light-emitting operation are sequentiallyperformed.

Next, the operation of the pixel 100 h illustrated in FIG. 38 isdescribed. The operation of the pixel 100 h illustrated in FIG. 38 canbe mainly divided into a first operation, a second operation, a thirdoperation, and a fourth operation. One operation is added to theoperations of the pixel 100 illustrated in FIG. 31A or the like. Thesecond operation, the third operation, and the fourth operation of thesemiconductor device 10 h illustrated in FIGS. 40B to 40D correspond tothe first operation, the second operation, and the third operation ofthe pixel 100 illustrated in FIG. 31A, respectively.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 38, FIG. 38 shows symbols representing thepotentials of nodes between elements and the potentials of wirings, asFIG. 29B does. The operation of the circuit configuration illustrated inFIG. 38 is explained with symbols of Vgs and Vc, as in FIG. 29B.

First, the first operation is described with reference to FIG. 40A. Notethat reference numerals of elements in FIG. 40A are omitted. The firstoperation is additionally provided to the operation of the pixel 100illustrated in FIG. 29A or the like. Note that in the drawings, aconduction state and a non-conduction state of the switches are denotedby ON and OFF. In addition, how Vgs, Vc, the potential of the nodeA, thepotential of the nodeB, the potential of the nodeC, the potential of thenodeD, the potential of the nodeE, the potential of the nodeF, and thepotential of the nodeG, which are illustrated in FIG. 29B, are appliedis described.

In the first operation, a potential for initialization is applied tosome extent at each node before initialization of the potential of eachnode (initialization before initialization). Specifically, the nodeG isset at Vinit and the nodeD is set at Vcat. The nodeA can be set at anypotential. In addition, the switch 104 and the switch 105 are turned on,and the switch 102 and the switch 103 are turned off. The nodeB and thenodeC are then set at Vinit or the potential ΔVinit which is close toVinit. The nodeE is set at Vy and the nodeF is set at Vinit. Note thatVgs and Vc are omitted because the first operation uses Vy which is asignal of an operation before the first operation.

The potential Vy is input before the first operation. The case where Vyenables the transistor 101 to operate as part of a current source isexplained. The potential Vy is set so that current flows between thefirst terminal and the second terminal of the transistor 101 in thefirst operation. Usually, Vinit is very low and accordingly thetransistor 101 is turned on because of Vy in many cases.

Therefore, in the first operation, the nodeF is set at Vinit and currentflows between the first terminal and the second terminal of thetransistor 101; as a result, the nodeB and the nodeC are set at Vinit orthe potential ΔVinit which is close to Vinit.

That is, the first operation decreases the potentials of the nodeB andthe nodeC. By the decrease in the potentials of the nodeB and the nodeCin the first operation, the following second operation can initializethe potential of each node at high speed. In particular, when thelight-emitting element 106 has large capacitance, the followingoperation can be performed smoothly by the decrease in the potentials ofthe nodeB and the nodeC in advance. Note that even if the potentials ofthe nodeB and the nodeC cannot be sufficiently decreased, it is not aproblem unless subsequent operations are adversely affected.

The second operation described with reference to FIG. 40B is the same asthe first operation described with reference to FIG. 31A and thereforethe description thereof is omitted.

In the second operation, the switch 102 and the switch 103 are turned onand the potential of the nodeG is set to VDD, and these operations canbe performed at the same time or at different timings.

It is preferable that, for example, the potential of the nodeG bechanged from Vinit to VDD before or at the same time as the switch 103is turned on. This is because the potential of the nodeE can beincreased easily in that case.

Then, the third operation illustrated in FIG. 40C is the same as thesecond operation described with reference to FIG. 31B and therefore thedescription thereof is omitted. Then, the fourth operation illustratedin FIG. 40D is the same as the third operation described with referenceto FIG. 31C and therefore the description thereof is omitted.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIGS. 40A to 40D in which thethreshold voltage of the transistor is corrected. In such a manner, avariety of circuits can be employed.

For example, specifically, the switch 102, the switch 103, the switch104, and the switch 105 can be provided any place and the numbersthereof is not limited as long as the switches can control a conductionstate and a non-conduction state between nodes. In the case of the firstoperation described with reference to FIG. 40A, a connection relationillustrated in FIG. 41A can be employed. In the case of the secondoperation described with reference to FIG. 40B, a connection relationillustrated in FIG. 41B can be employed. In the case of the thirdoperation described with reference to FIG. 40C, a connection relationillustrated in FIG. 41C can be employed. In the case of the fourthoperation described with reference to FIG. 40D, a connection relationillustrated in FIG. 41D can be employed. The potential of each node canhave any level unless the node affects the operations.

Note that the operation for correcting the threshold voltage of thetransistor is described with reference to FIGS. 40A to 40D but oneembodiment of the present invention is not limited thereto. For example,the operation for correcting variations in the mobility of thetransistor 101 may be performed between the third operation in FIG. 40Cand the fourth operation in FIG. 40D.

The operation for correcting the mobility of the transistor 101 isdescribed with reference to FIG. 42A.

The operation for correcting the mobility of the transistor 101 is thesame as the third operation described with reference to FIG. 33C and thedescription thereof is omitted.

In the operation for correcting the mobility of the transistor 101, thepotential of the gate of the transistor 101 becomes (Vsig+Vth−ΔVel)which is set in consideration of the mobility of the transistor 101.Accordingly, through the above operation, adverse effect of variationsin the mobility of the transistor on the amount of current supplied tothe light-emitting element can be reduced.

The location of the switch or the number of switches can be changed andappropriate voltage can be supplied so as to achieve the similaroperation to the operation described in FIG. 42A in which the mobilityof the transistor is corrected. In such a manner, a variety of circuitscan be employed.

For example, specifically, the switch 102, the switch 103, the switch104, and the switch 105 can be provided any place and the numbersthereof is not limited as long as the switches can control a conductionstate and a non-conduction state between nodes. In the case of theoperation for correcting the mobility of the transistor described withreference to FIG. 42A, a connection relation illustrated in FIG. 42B canbe employed.

The potential of the wiring 110 h is switched between Vinit and VDD inthe circuit configuration illustrated in FIG. 38, but anotherconfiguration can be used. For example, a configuration illustrated inFIG. 43 may be employed: a wiring 110A and a wiring 110B are providedinstead of the wiring 110 h, and Vinit is supplied to the wiring 110Aand VDD is supplied the wiring 110B. At this time, a switch 105Aprovided between the wiring 110A and the nodeF and a switch 105Bprovided between the wiring 110B and the nodeF may perform switching soas to achieve the similar operation to the operation described withreference to FIGS. 40A to 40D.

The case of FIG. 43 is further described with reference to FIG. 82. FIG.82 illustrates a circuit 113A connected to the wiring 110A in FIG. 43, acircuit 113B connected to the wiring 110B in FIG. 43, a wiring 135Aconnected to the switch 105A, a scan line driver circuit 202E connectedto the wiring 135A, a wiring 135B connected to the switch 105B, and ascan line driver circuit 202F connected to the wiring 135B.

The circuit 113A has a function of supplying Vinit and examples of thecircuit 113A are a power supply circuit and a voltage follower circuit.The circuit 113B has a function of supplying VDD and an example of thecircuit 113B is a power supply circuit. In addition, the switching ofthe switch 105A is controlled with a wiring 135A and the switching ofthe switch 105B is controlled with a wiring 135B. As an example, thewiring 135A and the wiring 135B are connected to a scan line drivercircuit 202E and a scan line driver circuit 202F, respectively. The scanline driver circuit 202E and the scan line driver circuit 202F each haveat least a function of supplying an H-level signal or an L-level signal.

In the case of the circuit configuration illustrated in FIG. 43 or FIG.82, pixels adjacent to each other in a column direction can sharewirings to be driven. Specifically, as illustrated in FIG. 83, whenattention is paid to the pixel 100 _(—) n and the pixel 100 _(—) n+1which are a pixel in the n-th row and a pixel in the (n+1)th row, whichhave the structures illustrated in FIG. 43 and FIG. 82, respectively,the structure in which the wiring 133 in the n-th row and the wiring135A in the (n+1)th row branches from a wiring connected to the scanline driver circuit can be used. With such a structure, the area of thewirings in the pixel region can be reduced.

The wirings explained with reference to FIG. 83 can be used in commonoutside the pixel region. Specifically, the following structureillustrated in FIG. 84 is also possible: the wiring from the scan linedriver circuit 202D branches outside the pixel region, and the branchedwirings function as the wiring 133 _(—) n of the pixel 100 _(—) n andthe wiring 135A_n+1 of the pixel 100 _(—) n+1. With such a structure,the number of output terminals of the scan line driver circuit 202D canbe reduced.

As described above, in the circuit configuration described in thisembodiment, initialization before initialization can be performed byswitching of the potential of the wiring 110 h between Vinit and VDD.Accordingly, the potential of each node can be initialized at highspeed. In the fourth operation, Vgs of the transistor 101 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 101. Accordingly, with this structure, adverse effect ofvariations in the threshold voltage of the transistor on the amount ofcurrent supplied to the load can be reduced.

Note that FIG. 43, or the like illustrates an example of a circuitconfiguration; accordingly, a transistor can be provided additionally.In each node in FIG. 43, or the like, it is possible not to provide anadditional transistor, switch, a passive element, or the like. Forexample, transistors directly connected to the nodeA, the nodeB, thenodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG are notadditionally provided. Accordingly, for example, the following structurecan be used: only the transistor 104T is directly connected to the nodeCand the other transistors are not directly connected to the nodeC.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 9

In this embodiment, an example of a configuration different from thecircuit configurations of the pixel of the display devices described inEmbodiments 7 and 8 is described.

FIG. 44 illustrates a pixel 100 p having a circuit configuration similarto the pixel 100 illustrated in FIG. 28A. The pixel 100 p illustrated inFIG. 44 is different from the pixel 100 illustrated in FIG. 28A in thatthe potential supplied to the wiring 108 is Vsig, a wiring 108 p and aswitch 102 p are provided, and Vinit is supplied from the wiring 108 p.Note that components in common with those in FIG. 28A are denoted bycommon reference numerals, and the description thereof is omitted.

A transistor can be applied to each of the switch 102, the switch 102 p,the switch 103, the switch 104, and the switch 105 in the pixel 100 pillustrated in FIG. 44. When an n-channel transistor is applied to eachof the switch 102, the switch 103, the switch 104, and the switch 105,as illustrated in FIG. 45, switching is controlled with the wirings 131to 134 and the wiring 131 p.

In FIG. 44, a first terminal of the switch 102 p is connected to thefirst terminal of the transistor 101, the first terminal of the switch102, and the first terminal of the switch 104. A second terminal of theswitch 102 p is connected to the wiring 108 p. The switching of theswitch 102 p is controlled with the wiring 131 p.

Note that in order to explain the operation of the pixel 100 pillustrated in FIG. 44, FIG. 45 shows symbols representing thepotentials of nodes between elements and the potentials of wirings. FIG.45 also shows Vgs between the one of the terminals (mainly serving as asource) and the gate of the transistor 101 and Vc between the electrodesof the capacitor 107.

A nodeA, a nodeB, a nodeC, a nodeD, a nodeE, a nodeF, a nodeG, and anodeH correspond to nodes and wirings illustrated in FIG. 45. Thepotential of the nodeA corresponds to the potential of the wiring 108.The potential of the nodeB corresponds to the potential of a wiringconnecting the first terminal of the transistor 101, the first terminalof the switch 102, the first terminal of the switch 104, and the firstterminal of the switch 102 p. The potential of the nodeC corresponds tothe potential of a wiring connecting the second terminal of the switch104, the one of the electrodes of the light-emitting element 106, andthe other of the electrodes of the capacitor 107. The potential of thenodeD corresponds to the potential of the wiring 109. The potential ofthe nodeE corresponds to the potential of a wiring connecting the gateof the transistor 101, the one of electrodes of the capacitor 107, andthe first terminal of the switch 103. The potential of the nodeFcorresponds to the potential of a wiring connecting the second terminalof the transistor 101, the second terminal of the switch 103, and thefirst terminal of the switch 105. The potential of the nodeG correspondsto the potential of the wiring 110. The potential of the nodeE the nodeHcorresponds to the potential of the wiring 108 p.

Here, a display device including the pixel 100 p is described withreference to a block diagram of FIG. 46.

The display device includes a signal line driver circuit 201, a scanline driver circuit 202A, a scan line driver circuit 202B, a scan linedriver circuit 202C, a scan line driver circuit 202D, a scan line drivercircuit 202E, a pixel region 203, and an initialization signal linedriver circuit 205. The pixel region 203 is provided with a plurality ofsignal lines S1 to Sn extended from the signal line driver circuit 201in a column direction; a plurality of signal lines Si1 to Sin extendedfrom the initialization signal line driver circuit 205 in a columndirection; a plurality of scan lines Ga1 to Gam extended from the scanline driver circuit 202A in a row direction; a plurality of scan linesGb1 to Gbm extended from the scan line driver circuit 202B in a rowdirection; a plurality of scan lines Gc1 to Gcm extended from the scanline driver circuit 202C in a row direction; a plurality of scan linesGd1 to Gdm extended from the scan line driver circuit 202D in a rowdirection; a plurality of scan lines Ge1 to Gem extended from the scanline driver circuit 202E in a row direction; a plurality of pixels 100 pprovided in matrix, connected to respective signal lines S1 to Sn, andconnected to respective scan lines Ga1 to Gam, Gb1 to Gbm, Gc1 to Gcm,Gd1 to Gdm, and Ge1 to Gem; and power supply lines P1 to Pn which areparallel to the signal lines S1 to Sn. The pixel 100 is connected to thesignal line Sj (one of the signal lines S1 to Sn), the initializationsignal line Sij (one of the initialization signal lines S1 to Sn), thescan line Gai (one of the scan lines Ga1 to Gam), the scan line Gbi (oneof the scan lines Gb1 to Gbm), the scan line Gci (one of the scan linesGc1 to Gcm), the scan line Gdi (one of the scan lines Gd1 to Gdm), thescan line Gei (one of the scan lines Ge1 to Gem), and the power supplyline Pj (one of the power supply lines P1 to Pn).

The scan line Gai corresponds to the wiring 131 in FIG. 45. The scanline Gbj corresponds to the wiring 132 in FIG. 45. The scan line Gcjcorresponds to the wiring 133 in FIG. 45. The scan line Gdj correspondsto the wiring 134 in FIG. 45. The scan line Gej corresponds to thewiring 131 p in FIG. 45. The signal line Sj corresponds to the wiring108 in FIG. 45. The initialization signal line Sij corresponds to thewiring 108 p in FIG. 45. The power supply line Pj corresponds to thewiring 110 in FIG. 45. Although not illustrated in FIG. 46, cathodelines each of which a plurality of pixels uses in common and the cathodeline corresponds to the wiring 109.

A scan line is selected with the use of a signal output from the scanline driver circuits 202A to 202E. The potential of each node of thepixels 100 connected to the selected scan line is initialized (firstoperation). Then, a video signal is written to the initialized pixel 100to obtain the threshold voltage of a transistor (second operation).After the threshold voltage of the transistor is obtained by writing ofthe video signal, the operation moves to light emission. The pixel emitslight in accordance with the video signal written to the pixel (thirdoperation). In this manner, the initialization of the pixel 100,obtaining of the threshold voltage, and a light-emitting operation aresequentially performed.

Next, the operation of the pixel 100 h illustrated in FIG. 44 isdescribed. The operation of the pixel 100 h illustrated in FIG. 44 canbe mainly divided into the first operation, the second operation, andthe third operation.

First, the first operation is described with reference to FIG. 47A. Notethat reference numerals of elements in FIG. 47A are omitted. Aconduction state and a non-conduction state of the switches are denotedby ON and OFF. In addition, how Vgs, Vc, the potential of the nodeA, thepotential of the nodeB, the potential of the nodeC, the potential of thenodeD, the potential of the nodeE, the potential of the nodeF, thepotential of the nodeG, and the potential of the nodeH, which areillustrated in FIG. 45, are applied is described.

The first operation initializes the potential of each node.Specifically, the nodeA is set at any potential, for example, Vsig, thenodeD is set at Vcat, the nodeG is set at VDD, and the nodeH is set atVinit. Then, the switch 102 p, the switch 103, the switch 104, and theswitch 105 are turned on, and the switch 102 is turned off. Thus, thenodeB is set at Vinit, the nodeC is set at Vinit, the nodeE is set atVDD, and the nodeF is set at VDD. Further, Vgs becomes (VDD−Vinit), andVc becomes (VDD−Vinit).

The first operation described with reference to FIG. 47A is differentfrom that of described with reference to FIG. 31A in Embodiment 7 inthat Vinit supplied to the nodeB and the nodeC is supplied from thewiring 108 p through the switch 102 p. With the structure,initialization can be performed without change in potential of thewiring 108 and the initialization of each node can be performed at highspeed. Alternatively, initialization of each node can be performed whilea potential is supplied from the wiring 108 to another pixel 100 pconnected to the wiring 108. Therefore, an operation period for theinitialization can be longer.

Next, the second operation is described with reference to FIG. 47B, asin FIG. 47A.

The second operation is the operation for obtaining the thresholdvoltage of the transistor 101 with the use of Vgs by discharging thepotential of the gate of the transistor 101 (or the electric charge ofthe capacitor 107). Specifically, the nodeA is set at Vsig, the nodeD isset at Vcat, the nodeG is set at VDD, and the nodeH is set at Vinitthough it can be any potential. Then, the switch 102 and the switch 103are turned on, and the switch 104, the switch 102 p, and the switch 105are turned off. Thus, the potential of the nodeB becomes Vsig, thepotential of the nodeC becomes (Vinit−Vx), the potential of the nodeEbecomes (Vsig+Vth), and the potential of the nodeF becomes (Vsig+Vth).Further, Vgs becomes Vth and Vc becomes (Vsig+Vth−Vinit+Vx).

The second operation described with reference to FIG. 47B is differentfrom that described in Embodiment 7 with reference to FIG. 31B in thatthe switch 102 p is turned off. Therefore, the second operation in thisembodiment is the same as the second operation described with referenceto FIG. 31B. By the second operation, the potential of the nodeEcorresponding to the potential of the gate of the transistor 101 can be(Vsig+Vth) which includes the threshold voltage of the transistor 101.

Note that in the second operation, the switch 104, the switch 105, andthe switch 102 p are turned off and the switch 102 is turned on, andthese operations can be performed at the same time or at differenttimings.

For example, it is preferable that the switch 102 be turned on at thesame time as or after the switch 102 p is turned off. This is because ashort circuit between the nodeA and the nodeH can be prevented easily.

Next, the third operation is described with reference to FIG. 47C, as inFIGS. 47A and 47B.

The third operation is the operation for outputting current to thelight-emitting element 106 with the use of the transistor 101 as part ofa current source. Specifically, the nodeA is set at Vsig though it canbe any potential, the nodeD is set at Vcat, the nodeG is set at VDD, andthe nodeH can be set at Vinit though it can be any potential. Then, theswitch 104 and the switch 105 are turned on, and the switch 102, theswitch 102 p, and the switch 103 are turned off. Then, the nodeB and thenodeC become Vel, the nodeE becomes (Vsig+Vth−Vinit+Vx+Vel), and thenodeF becomes VDD. In addition, Vgs becomes (Vsig+Vth−Vinit+Vx) and Vcbecomes (Vsig+Vth−Vinit+Vx).

The third operation described with reference to FIG. 47C is differentfrom that described in Embodiment 7 with reference to FIG. 31C in thatthe switch 102 p is turned off. Therefore, the third operation in thisembodiment is the same as the third operation described with referenceto FIG. 31C. By the third operation, Vgs of the transistor 101 becomes(Vsig+Vth−Vinit+Vx), which includes the threshold voltage of thetransistor 101. Accordingly, through the above operations, adverseeffect of variations in the threshold voltage of the transistor on theamount of current supplied to the light-emitting element can be reduced.

Note that FIG. 44 illustrates the circuit configuration of thisembodiment but one embodiment of the present invention is not limitedthereto. The location of the switch or the number of switches can bechanged and appropriate voltage can be supplied so as to achieve thesimilar operation to the operation described in FIGS. 47A to 47C inwhich the threshold voltage of the transistor is corrected. In such amanner, a variety of circuits can be employed.

For example, specifically, the switch 102, the switch 102 p, the switch103, the switch 104, and the switch 105 can be provided any place andthe numbers thereof is not limited as long as the switches can control aconduction state and a non-conduction state between nodes. In the caseof the first operation described with reference to FIG. 47A, aconnection relation illustrated in FIG. 48A can be employed. In the caseof the second operation described with reference to FIG. 47B, aconnection relation illustrated in FIG. 48B can be employed. In the caseof the third operation described with reference to FIG. 47C, aconnection relation illustrated in FIG. 48C can be employed. Thepotential of each node can have any level unless the node affects theoperations.

As described above, in the circuit configuration described in thisembodiment, the wiring 108 p is provided and the initialization can beperformed with the use of Vinit supplied from the wiring 108 p throughthe switch 102 p. Accordingly, time for initializing the potential ofeach node can be long. Alternatively, the initialization with the use ofVinit is not necessarily performed by using the wiring 108, which cansave time allowing time for the second operation to be longer. In thethird operation, Vgs of the transistor 101 becomes (Vsig+Vth−Vinit+Vx),which includes the threshold voltage of the transistor 101. Accordingly,with this structure, adverse effect of variations in the thresholdvoltage of the transistor on the amount of current supplied to the loadcan be reduced.

In the case of the circuit configuration illustrated in FIG. 44 or FIG.45, pixels adjacent to each other in a column direction can sharewirings to be driven. Specifically, as illustrated in FIG. 85, whenattention is paid to the pixel 100 _(—) n and the pixel 100 _(—) n+1which are a pixel in the n-th row and a pixel in the (n+1)th row, whichhave the structures illustrated in FIG. 44 and FIG. 45, respectively,the structure in which the wiring 133 in the n-th row and the wiring 131p in the (n+1)th row branches from a wiring connected to the scan linedriver circuit can be used. With such a structure, the area of thewirings in the pixel region can be reduced.

The wirings explained with reference to FIG. 85 can be used in commonoutside the pixel region. Specifically, the following structureillustrated in FIG. 86 is also possible: the wiring from the scan linedriver circuit 202D branches outside the pixel region, and the branchedwirings function as the wiring 133 _(—) n of the pixel 100 _(—) n andthe wiring 131 p _(—) n+1 of the pixel 100 _(—) n+1. With such astructure, the number of output terminals of the scan line drivercircuit 202D can be reduced.

Note that the operation for correcting mobility can be performed withthe use of the circuits illustrated in FIG. 44 and FIG. 45, as theoperations illustrated in FIG. 33C, FIG. 34C, FIG. 42A, and FIG. 42B.

Note that FIG. 44, or the like illustrates an example of a circuitconfiguration; accordingly, a transistor can be provided additionally.In each node in FIG. 44, or the like, it is possible not to provide anadditional transistor, switch, a passive element, or the like. Forexample, transistors directly connected to the nodeA, the nodeB, thenodeC, the nodeD, the nodeE, the nodeF, or/and the nodeG are notadditionally provided. Accordingly, for example, the following structurecan be used: only the transistor 104T is directly connected to the nodeCand the other transistors are not directly connected to the nodeC.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 10

The operations of the circuit configurations are described inEmbodiments 7 to 9 under the assumption that the parasitic capacitanceof the light-emitting element 106 is utilized; however, anotherconfiguration can be used. In this embodiment, a configuration in whicha capacitor is electrically connected in parallel to the light-emittingelement 106 provided in the circuit configuration in any of the aboveembodiments.

FIG. 49 illustrates a pixel 100C and is different from FIG. 28A in thata capacitor 107C is electrically connected in parallel to thelight-emitting element 106 connected to the pixel 100C. The capacitor107C can be connected to the wiring 110 as illustrated in FIG. 87.Alternatively, the capacitor 107C can be connected to another wiring.Further, FIG. 50 illustrates a pixel 100 hC and is different from FIG.43 in that the capacitor 107C is electrically connected in parallel tothe light-emitting element 106 connected to the pixel 100 hC. Thecapacitor 107C can be connected to the wiring 110A or the wiring 110B asillustrated in FIG. 88 or FIG. 89. Moreover, FIG. 51 illustrates a pixel100 pC and is different from FIG. 44 in that a capacitor 107C iselectrically connected in parallel to the light-emitting element 106connected to the pixel 100 hC. The capacitor 107C can be connected tothe wiring 108 p as illustrated in FIG. 90.

The capacitor 107C is electrically connected to the light-emittingelement 106 as illustrated in FIG. 49, FIG. 50, and FIG. 51, so thatvariations in electric charge at the nodeC can be small or Vx can be lowin the operation for initialization and the operation for obtainingthreshold voltage which are described in any of the above embodiments.When Vx can be low, the semiconductor device can supply a more accurateamount of current to the light-emitting element 106.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 11

In this embodiment, a configuration different from the circuitconfigurations of the pixels described in Embodiments 7 to 10 isdescribed.

FIG. 52 illustrates a pixel 100 h having a circuit configuration similarto the pixel 100 illustrated in FIG. 29B. The pixel 100 hm illustratedin FIG. 52 is different from the pixel 100 illustrated in FIG. 29B inthat the pixel 100 hm is connected to a wiring 109 m. The potential Vupor Vcat is supplied to the wiring 109 m, switching the potentials asnecessary. Note that components in common with those in FIG. 29B aredenoted by common reference numerals, and the description thereof isomitted.

The potential Vup can be higher than Vcat. The potential Vup is high, sothat Vinit is prevented from being too low.

Here, a display device including the pixel 100 hm is described withreference to a block diagram of FIG. 53.

The display device includes a signal line driver circuit 201, a scanline driver circuit 202A, a scan line driver circuit 202B, a scan linedriver circuit 202C, a scan line driver circuit 202D, a pixel region203, and a cathode line driver circuit 206. The pixel region 203 isprovided with a plurality of signal lines S1 to Sn extended from thesignal line driver circuit 201 in a column direction; a plurality ofscan lines Ga1 to Gam extended from the scan line driver circuit 202A ina row direction; a plurality of scan lines Gb1 to Gbm extended from thescan line driver circuit 202B in a row direction; a plurality of scanlines Gc1 to Gcm extended from the scan line driver circuit 202C in arow direction; a plurality of scan lines Gd1 to Gdm extended from thescan line driver circuit 202D in a row direction; a plurality of cathodelines C1 to Cm extended from the cathode line driver circuit 206 in arow direction; a plurality of pixels 100 hm provided in matrix,connected to respective signal lines S1 to Sn, and connected torespective scan lines Ga1 to Gam, Gb1 to Gbm, Gc1 to Gcm, and Gd1 toGdm; and power supply lines P1 to Pn which are parallel to the signallines S1 to Sn. The pixel 100 is connected to the signal line Sj (one ofthe signal lines S1 to Sn), the scan line Gai (one of the scan lines Ga1to Gam), the scan line Gbi (one of the scan lines Gb1 to Gbm), the scanline Gci (one of the scan lines Gc1 to Gcm), the scan line Gdi (one ofthe scan lines Gd1 to Gdm), the cathode line Ci (one of cathode lines C1to Cm) and the power supply line Pj (one of the power supply lines P1 toPn).

The scan line Gai corresponds to the wiring 131 in FIG. 52. The scanline Gbj corresponds to the wiring 132 in FIG. 52. The scan line Gcjcorresponds to the wiring 133 in FIG. 52. The scan line Gdj correspondsto the wiring 134 in FIG. 52. The signal line Sj corresponds to thewiring 108 in FIG. 52. The power supply line Pj corresponds to thewiring 110 in FIG. 52. The cathode line Ci corresponds to the wiring 109in FIG. 52.

A scan line is selected with the use of a signal output from the scanline driver circuits 202A to 202D. The potential of each node of thepixels 100 hm connected to the selected scan line is initialized (firstoperation). Then, a video signal is written to the initialized pixel 100hm to obtain the threshold voltage of a transistor (second operation).After the threshold voltage of the transistor is obtained by writing ofthe video signal, the operation moves to light emission. The pixel emitslight in accordance with the video signal written to the pixel (thirdoperation). In this manner, the initialization of the pixel 100 hm,obtaining of the threshold voltage, and light-emitting operation aresequentially performed.

Next, the operation of the pixel 100 hm illustrated in FIG. 52 isdescribed. The operation of the pixel 100 hm illustrated in FIG. 52 canbe mainly divided into a first operation, a second operation, and athird operation.

Note that in order to explain the operation of the circuit configurationillustrated in FIG. 52, FIG. 52 shows symbols representing thepotentials of nodes between elements and the potentials of wirings, asFIG. 29B does. The operation of the circuit configuration illustrated inFIG. 52 is explained with symbols of Vgs and Vc, as in FIG. 29B.

The first operation illustrated in FIG. 54A is the same as the firstoperation described with reference to FIG. 31A except that the nodeD isset at Vup. The description of the same portions is omitted. When thenodeD is set at Vup, current flowing to the light-emitting element 106in the first operation can be reduced more surely. Alternatively, anormal operation can be performed with ease without making Vinitextremely low. Therefore, another potential can have smaller amplitude,resulting in reduction in power consumption.

The potential Vup is higher than Vinit and Vsig. Alternatively, Vup isapproximately equal to Vinit. Note that the potential is preferably setso as not to cause dielectric breakdown of the light-emitting element106.

The second operation illustrated in FIG. 54B is the same as the secondoperation described with reference to FIG. 31B except that the nodeD isset at Vup. The description of the same portions is omitted. When thenodeD is set at Vup, current flowing to the light-emitting element 106in the second operation can be reduced more surely.

Then, the third operation illustrated in FIG. 54C is the same as thethird operation described with reference to FIG. 31C and therefore thedescription thereof is omitted. Note that the third operationillustrated in FIG. 54C is different from the first operation describedwith reference to FIG. 54A and the second operation described withreference to FIG. 54B in that the nodeD is set at Vcat and current flowsthrough the load.

With the structure described with reference to FIGS. 54A to 54C, onlywhen the transistor 101 is completely set to allow the semiconductordevice to serve as a current source, current can flow without causingmalfunction.

Note that the operation for correcting mobility can be performed withthe use of the circuits illustrated in FIG. 52, as the circuitsillustrated in FIG. 33C, FIG. 34C, FIG. 42A, and FIG. 42B.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 12

In this embodiment, structures of a top view and a cross-sectional viewcorresponding to the circuit diagram of the pixel of the display deviceillustrated in FIG. 28A of Embodiment 7 are described.

A top view of FIG. 55 shows a structure described in Embodiment 7 withreference to FIG. 28A. In the top view of FIG. 55, each transistor is aninverted staggered transistor.

As a structure corresponding to that in FIG. 28A, the top view of FIG.55 of a pixel which can be applied to the display device shows thetransistor 101, the switch 102, the switch 103, the switch 104, theswitch 105, the light-emitting element 106 (only one of electrodes isillustrated), the capacitor 107, the wiring 108, the wiring 110, thewiring 131, the wiring 132, the wiring 133, and the wiring 134.

The structure illustrated in FIG. 55 includes a conductive layer 851, asemiconductor layer 852, a conductive layer 853, a conductive layer 854,a conductive layer 855, a contact hole 856, a contact hole 857, and acontact hole 858. Note that an insulating layer in each layer is notillustrated here.

The conductive layer 851 has a region that functions as a gate electrodeor a scan line. Note that the conductive layer 851 is formed over asubstrate over which elements such as a transistor are provided. A baseinsulating layer may be sandwiched between the substrate and theconductive layer 851.

Although there is no particular limitation on a substrate used as thesubstrate, a glass substrate is preferably used. The base insulatinglayer has a function of preventing diffusion of an impurity element fromthe substrate, and can be formed to have a single-layer structure or alayered structure including any of a silicon nitride layer, a siliconoxide layer, a silicon nitride oxide layer, and a silicon oxynitridelayer.

As the substrate, a semiconductor substrate (e.g., a single crystalsubstrate or a silicon substrate), an SOI substrate, a quartz substrate,a plastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, paper including a fibrous material, a base material film, or thelike can be used, for example. As an example of a glass substrate, abarium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, soda lime glass substrate, and the like can be given. For aflexible substrate, a flexible synthetic resin such as plastics typifiedby polyethylene terephthalate (PET), polyethylene naphthalate (PEN), andpolyether sulfone (PES), or acrylic can be used, for example. For anattachment film, polypropylene, polyester, polyvinyl fluoride, polyvinylchloride, or the like can be used, for example. For a base materialfilm, polyester, polyamide, polyimide, an inorganic vapor depositionfilm, paper, or the like can be used, for example. Specifically, when atransistor is formed using a semiconductor substrate, a single crystalsubstrate, an SOI substrate, or the like, it is possible to form atransistor with few variations in characteristics, size, shape, or thelike and with high current supply capability and a small size. Byforming a circuit with the use of such a transistor, power consumptionof the circuit can be reduced or the circuit can be highly integrated.

Note that the transistor may be formed using one substrate, and then,the transistor may be transferred to another substrate. In addition tothe above substrates over which the transistor can be formed, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, a rubber substrate, orthe like can be used as a substrate to which the transistor istransferred. By using such a substrate, a transistor with excellentproperties or a transistor with low power consumption can be formed, adevice with high durability or high heat resistance can be formed, orreduction in weight or thickness can be achieved.

The conductive layer 851 can be formed to have a single-layer structureor a stacked-layer structure using a metal material such as molybdenum(Mo), titanium (Ti), chromium (Cr), tantalum (Ta), tungsten (W),aluminum (Al), copper (Cu), neodymium (Nd), or scandium (Sc), or analloy material including any of these as a main component.

The semiconductor layer 852 has a region functioning as a semiconductorlayer of the transistor.

The semiconductor layer 852 may include amorphous silicon. Thesemiconductor layer 852 may include polycrystalline silicon.Alternatively, The semiconductor layer 852 may include an organicsemiconductor, an oxide semiconductor, or the like.

The conductive layer 853 has regions functioning as wirings and sourceand drain of the transistor.

The conductive layer 853 can be formed using an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, an alloy including any of these elementsas a main component, an alloy film including a combination of any ofthese elements, or the like. The conductive film may have a structure inwhich a high-melting-point metal layer of Ti, Mo, W, or the like isstacked over and/or below a metal layer of Al, Cu, or the like. When anAl material to which an element (e.g., Si, Nd, or Sc) which preventsgeneration of hillocks and whiskers in an Al film is added is used, heatresistance can be increased.

Alternatively, the conductive layer 853 may be formed using a conductivemetal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide (In₂O₃—SnO₂,abbreviated to ITO), indium oxide-zinc oxide (In₂O₃—ZnO), or any ofthese metal oxide materials containing silicon oxide can be used.

The conductive layer 854 has a region functioning as a wiring. Note thatthe conductive layer 854 is provided to improve the planarity of aninsulating layer formed later to be in contact with a transparentconductive layer and is not necessarily provided.

The conductive layer 855 has a region functioning as one of theelectrodes of the light-emitting element. The conductive layer 855 has afunction of reflecting light in the case where light emitted from thelight-emitting element is obtained from the counter substrate side. Onthe other hand, the conductive layer 855 has a function of transmittinglight in the case where light emitted from the light-emitting element isobtained from the element substrate side.

The contact holes 856 each have a function of connecting the conductivelayer 851 and the conductive layer 853. An insulating layer functioningas a gate insulating layer is sandwiched between the conductive layer851 and the conductive layer 853. The insulating layer functioning asthe gate insulating layer can be formed to have a single-layer structureor a layered structure of a silicon oxide layer, a silicon nitridelayer, a silicon oxynitride layer, a silicon nitride oxide layer, analuminum oxide layer, an aluminum nitride layer, an aluminum oxynitridelayer, an aluminum nitride oxide layer, or a hafnium oxide layer byplasma-enhanced CVD, sputtering, or the like.

The contact hole 857 has a function of connecting the conductive layer853 and the conductive layer 854. An insulating layer functioning as apassivation layer is sandwiched between the conductive layer 853 and theconductive layer 854. For the passivation layer, an inorganic insulatingfilm such as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

The contact hole 858 has a function of connecting the conductive layer854 and the conductive layer 855. An insulating layer providing theplanarity of a surface is sandwiched between the conductive layer 854and the conductive layer 855. For the insulating layer providing theplanarity of the surface, an organic material such as polyimide, acrylicresin, or benzocyclobutene-based resin can be used. Other than suchorganic materials, it is also possible to use a low-dielectric constantmaterial (low-k material) or the like.

Next, structures of a cross section (taken along the chain double-dashedline A-A′ in FIG. 55) of a transistor functioning as the switch 105 andstructures of a cross section (taken along the chain double-dashed lineB-B′ in FIG. 55) of the capacitor 107 which is shown in the top view ofFIG. 55 are described with reference to FIGS. 91A and 91B.

The transistor functioning as the switch 105 in FIG. 91A is abottom-gate transistor and also referred to as an inverted staggeredtransistor, for example. There is no particular limitation on thestructure of the transistor; for example, a staggered type transistor ora planar type transistor having a top-gate structure or a bottom-gatestructure can be employed. Further, the transistor may have a singlegate structure including one channel formation region, a double gatestructure including two channel formation regions, or a triple gatestructure including three channel formation regions. Alternatively, thetransistor may have a dual gate structure including two gate electrodelayers positioned over and below a channel region with a gate insulatinglayer provided therebetween.

The transistor functioning as the switch 105 illustrated in FIG. 91Aincludes, over a substrate 400, the conductive layer 851 functioning asa gate, an insulating layer 401 functioning as a gate insulating layer,the semiconductor layer 852, and the conductive layers 853 functioningas a source and a drain. An insulating layer 402 is provided as apassivation layer so as to cover the transistor functioning as theswitch 105. An insulating layer 403 providing the planarity of thesurface is formed over the insulating layer 402.

The capacitor 107 illustrated in FIG. 91B includes, over the substrate400, the conductive layer 851 functioning as one of the electrodes, theinsulating layer 401, the semiconductor layer 852, and the conductivelayer 853 functioning as the other of the electrodes. An insulatinglayer 402 is provided as a passivation layer so as to cover thecapacitor 107. An insulating layer 403 providing the planarity of thesurface is formed over the insulating layer 403.

Note that a top view of the pixel which can be applied to the displaydevice illustrated in FIG. 55 is not limited to the above top view, andanother structure can be used.

As an example, a top view of the pixel described with reference to FIG.28A can be a top view illustrated in FIG. 56. FIG. 56 is different fromFIG. 55 in that the sources and the drains which are provided tosandwich channels of the transistors forming pixels are provided to bein the same direction. With the structure, variations in characteristicsof the transistors forming the pixels can be reduced.

The structure of a top view can also be a top view illustrated in FIG.57. FIG. 57 is different from FIG. 55 in that the size of the transistor101 which allows the semiconductor device to function as a currentsource is larger than the size of the transistor functioning as aswitch. With the structure, the amount of current flowing through thetransistor 101 which allows the semiconductor device to function as acurrent source can be increased.

The structure of a top view can also be a top view illustrated in FIG.58. FIG. 58 is different from FIG. 55 in that the other of the terminalsof the transistor 101 which allows the semiconductor device to functionas a current source is provided to have a U-shape and cover the one ofthe terminals of the transistor 101. With the structure, the amount ofcurrent flowing through the transistor 101 which allows thesemiconductor device to function as a current source can be increased.

The structure of a top view can also be a top view illustrated in FIG.59. FIG. 59 is different from FIG. 55 in that the one of the terminalsof the transistor 101 which allows the semiconductor device to functionas a current source is provided to have a U-shape and cover the other ofthe terminals of the transistor 101. With the structure, the amount ofcurrent flowing through the transistor 101 which allows thesemiconductor device to function as a current source can be increased.Further, parasitic capacitance when the potential of the gate of thetransistor 101 is increased by capacitive coupling can be large.

The structure of a top view can also be a top view illustrated in FIG.60. FIG. 60 is different from FIG. 55 in that the gate electrode of thetransistor forming the pixel is provided to cover the channel formationregion of the transistor. With the structure, light incident to thechannel formation region can be reduced and therefore lightdeterioration in characteristics of the transistor can be reduced.

Note that when the pixels described with reference to FIG. 28A havelight-emitting elements emitting light of different colors and arearranged, the sizes of the transistors 101 which allow the semiconductordevice to function as a current source can be different depending on thecolor. FIG. 61 is a top view of the structure in which the sizes of thetransistors 101 which allow the semiconductor device to function as acurrent source are changed every color. A transistor 101R in FIG. 61 isa transistor which allows the semiconductor device to function as acurrent source, in a pixel including a red light emitting element. Atransistor 101G in FIG. 61 is a transistor which allows thesemiconductor device to function as a current source, in a pixelincluding a green light emitting element. A transistor 101B in FIG. 61is a transistor which allows the semiconductor device to function as acurrent source, in a pixel including a blue light emitting element. Withthe structure, the proper amount of current of the light-emittingelements emitting light of colors can be supplied.

Note that when the pixels each described with reference to FIG. 28A havelight-emitting elements with different colors and are arranged, thewidths of the wirings 110 each functioning as a power source line can bedifferent depending on colors. FIG. 62 is a top view of the structure ofthe wirings 110 each functioning as a power source line, whose widthsare different depending on colors. A wiring 110R in FIG. 62 is a wiringfor supplying current to a red light emitting element. A wiring 110G inFIG. 62 is a wiring for supplying current to a green light emittingelement. A wiring 110B in FIG. 62 is a wiring for supplying current to ablue light emitting element. With the structure, the proper amount ofcurrent of light-emitting elements emitting each color of light can besupplied.

The above top view illustrates an inverted staggered transistor as eachtransistor, but the transistors may be top-gate transistors. FIG. 63 isa top view where each transistor forming the pixel is a top-gatetransistor. In that case, contact holes 859 are added in comparison withthe top view illustrated in FIG. 55.

The contact holes 859 each have a function of connecting thesemiconductor layer 852 and the conductive layer 853.

In the case where the transistor forming the pixel is a top-gatetransistor as illustrated in FIG. 63, a semiconductor layer of thetransistor is preferably formed using amorphous silicon orpolycrystalline silicon. With the structure, the semiconductor layer canbe used as a wiring between the transistors in such a manner that animpurity element such as phosphorus or boron is added to thesemiconductor layer to increase the conductivity of the semiconductorlayer.

Next, structures of a cross section (taken along the chain double-dashedline A-A′ in FIG. 63) of a transistor functioning as the switch 105which is shown in the top view of FIG. 63 and structures of a crosssection (taken along the chain double-dashed line B-B′ in FIG. 63) ofthe capacitor 107 are described with reference to FIGS. 92A and 92B.

The transistor functioning as the switch 105 in FIG. 92A is a top-gatetransistor, for example. Further, the transistor may have a single gatestructure including one channel formation region, a double gatestructure including two channel formation regions, or a triple gatestructure including three channel formation regions. Alternatively, thetransistor may have a dual gate structure including two gate electrodelayers positioned over and below a channel region with a gate insulatinglayer provided therebetween.

The transistor functioning as the switch 105 illustrated in FIG. 92Aincludes, over the substrate 410, the semiconductor layer 852 includingimpurity regions 852 _(—) n to which an impurity is added; an insulatinglayer 411 functioning as a gate insulating layer; the conductive layer851 functioning as a gate; an insulating layer 412 functioning as aninterlayer insulating layer; and the conductive layers 853 functioningas a source and a drain. An insulating layer 413 providing the planarityof the surface is formed to cover the insulating layer 412 and theconductive layer 853.

The capacitor 107 illustrated in FIG. 92B includes, over the substrate410, the semiconductor layer 852 including impurity regions 852 _(—) nto each of which an impurity is added; an insulating layer 411functioning as a gate insulating layer; and the conductive layer 851functioning as the other of the electrodes. The impurity regions 852_(—) n function as the one of the electrodes. The conductive layer 853connected to the semiconductor layer 852 through a contact hole providedin the insulating layer 411 and the insulating layer 412 is provided. Aninsulating layer 413 providing the planarity of the surface is formed tocover the insulating layer 412 and the conductive layer 853.

FIG. 64 is a top view illustrating a structure in which thesemiconductor layer is used as a wiring between the transistors in sucha manner that the semiconductor layer is amorphous silicon orpolycrystalline silicon and an impurity element such as phosphorus orboron is added to the semiconductor layer to increase the conductivityof the semiconductor layer. A semiconductor layer 860 in FIG. 64 is aconductive layer whose conductivity is increased by addition of animpurity element.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 13

In the above embodiment, a transistor forming the pixel of the displaydevice is an n-channel transistor. On the other hand, in thisembodiment, a circuit configuration in which a p-channel transistor isused for the pixel of the display device is described.

The transistor 101 of the pixel 100 in FIG. 28A is an n-channeltransistor, but the transistor of the pixel can be a p-channeltransistor as illustrated in FIG. 65 (p-channel transistor 501 of apixel 500).

As shown by comparison between FIG. 28A and FIG. 65, a light-emittingelement is connected to make current flow in a direction opposite tothat in the case of the light-emitting element 106. Specifically, alight-emitting element 506 may be connected as shown in FIG. 65.

In FIG. 28A, the wiring 109 and the wiring 110 are supplied with Vcatand VDD, respectively. In FIG. 65, the potentials can be switched;specifically, the wiring 109 and the wiring 110 are supplied with VDDand Vcat, respectively. The potential Vinit for initializing thepotential of each node in the pixel can be higher than VDD and Vcat.

As described above, the transistor which allows the semiconductor deviceto function as a current source can be a p-channel transistor.

Note that each switch forming the pixel 100 in FIG. 28A can be ap-channel transistor. Specifically, as described in FIG. 66, atransistor 502T, a transistor 503T, a transistor 504T, and a transistor505T, which are p-channel transistors, may be used as the switches andthe pixel is controlled by the switching of the p-channel transistors. Asignal for switching the transistors may be supplied to the wirings 131to 134 in such a manner that the operation of the pixel is the same asthe operation described with reference to FIGS. 31A to 31C.

Note that in FIG. 28A, it is also possible to employ the structure inwhich each switch of the pixel 100 is an n-channel transistor and onlythe transistor which allows the semiconductor device to function as acurrent source is a p-channel transistor. Specifically, as illustratedin FIG. 67, each switch may be an n-channel transistor and only thetransistor which allows the semiconductor device to function as acurrent source is a p-channel transistor.

Note that the switches of the pixel have the same polarity in each ofFIG. 66 and FIG. 67, but the switches can have different polarities fromeach other. Specifically as illustrated in FIG. 68, the switches of thepixel 500 can include the p-channel transistor 502T, the n-channeltransistor 103T, the n-channel transistor 104T, and the p-channeltransistor 505T.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 14

In the above embodiments, transistors forming the pixels of the displaydevices are n-channel transistors in many cases. In particular, the casewhere a transistor which includes a channel formation region in an oxidesemiconductor layer is used in the circuit configuration of the pixel ofthe display device is described in this embodiment.

In FIG. 28A, the transistor 101 of the pixel 100 is a simple n-channeltransistor, but the transistor can be a transistor which includes achannel formation region in an oxide semiconductor layer, as illustratedin FIG. 69 (transistor 601 of a pixel 600). Note that a transistor whichincludes a channel formation region in an oxide semiconductor layer islabeled as OS in the figures, similarly to the transistor 601 in FIG.69.

In the structure in FIG. 69, the transistor 601 is a transistor whichincludes a channel formation region in an oxide semiconductor layer andtherefore off-state current of the transistor can be reduced.Accordingly, the pixel can have a circuit configuration which does noteasily allow malfunction.

Each switch forming the pixel 600 can be a transistor which includes achannel formation region in an oxide semiconductor layer. Specifically,as illustrated in FIG. 70, the switches can be transistors 602 to 605each of which includes a channel formation region in an oxidesemiconductor layer.

Note that in this specification, the off-state current is current thatflows between a source and a drain when a transistor is off. In the caseof an n-channel transistor (whose threshold voltage is, for example,about 0 to 2 V), off-state current refers to current flowing between thesource and the drain when negative voltage is applied between the gateand the source.

Next, a material of an oxide semiconductor layer in which a channelformation region is provided is described below. As described above, astructure in this embodiment may include a layer formed using an oxidesemiconductor (an oxide semiconductor layer), for example.

Examples of an oxide semiconductor include a four-component metal oxidesuch as an In—Sn—Ga—Zn—O-based oxide semiconductor; three-componentmetal oxides such as an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxidesemiconductor, and a Hf—In—Zn—O-based oxide semiconductor; two-componentmetal oxides such as an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, and anIn—Ga—O-based oxide semiconductor; and the like. In addition, any of theabove oxide semiconductors may contain an element other than In, Ga, Sn,and Zn, for example, SiO₂.

For example, an In—Sn—Zn—O-based oxide semiconductor means an oxidesemiconductor containing indium (In), tin (Sn), and zinc (Zn), and thereis no limitation on the composition ratio. For example, anIn—Ga—Zn—O-based oxide semiconductor means an oxide semiconductorcontaining indium (In), gallium (Ga), and zinc (Zn), and there is nolimitation on the composition ratio. An In—Ga—Zn—O-based oxidesemiconductor can be referred to as IGZO.

The oxide semiconductor layer can be formed using an oxide semiconductorfilm. In the case where an In—Sn—Zn—O-based oxide semiconductor film isformed by sputtering, a target which has a composition ratio ofIn:Sn:Zn=1:2:2, 2:1:3, 1:1:1, 20:45:35, or the like in an atomic ratiois used.

In the case where an In—Zn—O-based oxide semiconductor film is formed bysputtering, a target has a composition ratio of In:Zn=50:1 to 1:2 in anatomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferablyIn:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molarratio), more preferably In:Zn=1.5:1 to 15:1 in an atomic ratio(In₂O₃:ZnO=3:4 to 15:2 in a molar ratio). For example, in a target whichhas an atomic ratio of In:Zn:O=X:Y:Z, an inequality of Z>1.5X+Y issatisfied.

In the case where an In—Ga—Zn—O-based oxide semiconductor film is formedby sputtering, a target can have a composition ratio ofIn:Ga:Zn=1:1:0.5, 1:1:1, or 1:1:2 in an atomic ratio.

When the purity of the target is set to 99.99% or higher, alkali metal,hydrogen atoms, hydrogen molecules, water, a hydroxyl group, hydride, orthe like mixed to the oxide semiconductor film can be reduced. Inaddition, when the target is used, the concentration of alkali metalsuch as lithium, sodium, or potassium can be reduced in the oxidesemiconductor film.

Note that it has been pointed out that an oxide semiconductor isinsensitive to impurities, there is no problem when a considerableamount of metal impurities is contained in the film, and therefore,soda-lime glass which contains a large amount of alkali metal such assodium (Na) and is inexpensive can be used (Kamiya, Nomura, and Hosono,“Carrier Transport Properties and Electronic Structures of AmorphousOxide Semiconductors: The present status”, KOTAI BUTSURI (SOLID STATEPHYSICS), 2009, Vol. 44, pp. 621-633). But such consideration is notappropriate. Alkali metal is not an element included in an oxidesemiconductor, and therefore, is an impurity. Also, alkaline earth metalis impurity in the case where alkaline earth metal is not included in anoxide semiconductor. Alkali metal, in particular, Na becomes Na⁺ when aninsulating film in contact with the oxide semiconductor layer is anoxide and Na diffuses into the insulating layer. Further, in the oxidesemiconductor layer, Na cuts or enters a bond between metal and oxygenwhich are included in an oxide semiconductor. As a result, for example,deterioration of characteristics of the transistor, such as anormally-on state of the transistor due to shift of a threshold voltagein the negative direction, or reduction in mobility, occurs. Inaddition, variations in characteristics also occurs. Such deteriorationof characteristics and variations in characteristics of the transistordue to the impurity remarkably appear when the concentration of hydrogenin the oxide semiconductor layer is extremely low. Therefore, when thehydrogen concentration in the oxide semiconductor layer is less than orequal to 1×10¹⁸/cm³, preferably less than or equal to 1×10¹⁷/cm³, theconcentration of the above impurity is preferably reduced. Specifically,a measurement value of a Na concentration by secondary ion massspectrometry is preferably less than or equal to 5×10¹⁶/cm³, morepreferably less than or equal to 1×10¹⁶/cm³, still more preferably lessthan or equal to 1×10¹⁵/cm³. In a similar manner, a measurement value ofa Li concentration is preferably less than or equal to 5×10¹⁵/cm³, morepreferably less than or equal to 1×10¹⁵/cm³. In a similar manner, ameasurement value of a K concentration is preferably less than or equalto 5×10¹⁵/cm³, more preferably less than or equal to 1×10¹⁵/cm³.

The oxide semiconductor film is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor layer is preferably a CAAC-OS (c-axis alignedcrystalline oxide semiconductor) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal parts are includedin an amorphous phase. Note that in most cases, the crystal part fitsinside a cube whose one side is less than 100 nm. From an observationimage obtained with a transmission electron microscope (TEM), a boundarybetween an amorphous part and a crystal part in the CAAC-OS film is notclear. Further, with the TEM, a grain boundary in the CAAC-OS film isnot found. Thus, in the CAAC-OS film, a reduction in electron mobility,due to the grain boundary, is prevented.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by deposition or by performingtreatment for crystallization such as heat treatment after deposition.

With use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Thus, the transistor has highreliability.

An example of a crystal structure of the CAAC-OS film is described indetail with reference to FIGS. 71A to 71E, FIGS. 72A to 72C, FIGS. 73Ato 73C, and FIGS. 74A and 74B. In FIGS. 71A to 71E, FIGS. 72A to 72C,FIGS. 73A to 73C, and FIGS. 74A and 74B, the vertical directioncorresponds to the c-axis direction and a plane perpendicular to thec-axis direction corresponds to the a-b plane, unless otherwisespecified. When the expressions “an upper half” and “a lower half” aresimply used, they refer to an upper half above the a-b plane and a lowerhalf below the a-b plane (an upper half and a lower half with respect tothe a-b plane). Furthermore, in FIGS. 71A to 71E, O surrounded by acircle represents tetracoordinate O and O surrounded by a double circlerepresents tricoordinate O.

FIG. 71A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 71A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 71A. In the small group illustrated in FIG. 71A,electric charge is 0.

FIG. 71B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of an upper half and alower half in FIG. 71B. An In atom can also have the structureillustrated in FIG. 71B because an In atom can have five ligands. In thesmall group illustrated in FIG. 71B, electric charge is 0.

FIG. 71C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 71C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 71C. In thesmall group illustrated in FIG. 71C, electric charge is 0.

FIG. 71D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 71D,three tetracoordinate O atoms exist in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 71D, electric charge is +1.

FIG. 71E illustrates a small group including two Zn atoms. In FIG. 71E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 71E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 71A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom in FIG. 71B has oneproximate Ga atom in the downward direction, and the one O atom in thelower half has one proximate Ga atom in the upward direction. The one Oatom in the upper half with respect to the tetracoordinate Zn atom inFIG. 71C has one proximate Zn atom in the downward direction, and thethree O atoms in the lower half each have three proximate Zn atoms inthe upward direction. In this manner, the number of the tetracoordinateO atoms above the metal atom is equal to the number of the metal atomsproximate to and below each of the tetracoordinate O atoms. Similarly,the number of the tetracoordinate O atoms below the metal atom is equalto the number of the metal atoms proximate to and above each of thetetracoordinate O atoms. Since the coordination number of thetetracoordinate O atom is 4, the sum of the number of the metal atomsproximate to and below the O atom and the number of the metal atomsproximate to and above the O atom is 4. Accordingly, when the sum of thenumber of tetracoordinate O atoms above a metal atom and the number oftetracoordinate O atoms below another metal atom is 4, the two kinds ofsmall groups including the metal atoms can be bonded. The reason isdescribed as follows. For example, in the case where the hexacoordinatemetal (In or Sn) atom is bonded through three tetracoordinate O atoms inthe lower half, it is bonded to the pentacoordinate metal (Ga or In)atom or the tetracoordinate metal (Zn) atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded toanother metal atom through a tetracoordinate O atom in the c-axisdirection. In addition to the above, a medium group can be formed in adifferent manner by combining a plurality of small groups so that thetotal electric charge of the layered structure is 0.

FIG. 72A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 72B illustrates a largegroup including three medium groups. Note that FIG. 72C illustrates anatomic arrangement in the case where the layered structure in FIG. 72Bis observed from the c-axis direction.

In FIG. 72A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atom is illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 72A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 72A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 72A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge in a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, the small group including two Zn atoms as illustrated in FIG. 71Ecan be given. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 72B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above-described rule also applies to the following materials: afour-component metal oxide such as an In—Sn—Ga—Zn—O-based oxide; athree-component metal oxide such as an In—Ga—Zn—O-based oxide (alsoreferred to as IGZO), an In—Al—Zn—O—based oxide, a Sn—Ga—Zn—O-basedoxide, an Al—Ga—Zn—O-based oxide, a Sn—Al—Zn—O-based oxide, anIn—Hf—Zn—O-based oxide, an In—La—Zn—O-based oxide, an In—Ce—Zn—O-basedoxide, an In—Pr—Zn—O-based oxide, an In—Nd—Zn—O-based oxide, anIn—Sm—Zn—O-based oxide, an In—Eu—Zn—O-based oxide, an In—Gd—Zn—O-basedoxide, an In—Tb—Zn—O-based oxide, an In—Dy—Zn—O-based oxide, anIn—Ho—Zn—O-based oxide, an In—Er—Zn—O-based oxide, an In—Tm—Zn—O-basedoxide, an In—Yb—Zn—O-based oxide, or an In—Lu—Zn—O-based oxide; atwo-component metal oxide such as an In—Zn—O-based oxide, aSn—Zn—O-based oxide, an Al—Zn—O-based oxide, a Zn—Mg—O-based oxide, aSn—Mg—O-based oxide, an In—Mg—O-based oxide, or an In—Ga—O-based oxide;and the like.

As an example, FIG. 73A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 73A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups are bonded, sothat a large group is formed.

FIG. 73B illustrates a large group including three medium groups. Notethat FIG. 73C illustrates an atomic arrangement in the case where thelayered structure in FIG. 73B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, and +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be formed using not only the medium group illustratedin FIG. 73A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 73A.

When the large group shown in FIG. 73B is repeated, a crystal of anIn—Ga—Zn—O system can be obtained. Note that a layered structure of theobtained In—Ga—Zn—O-based crystal can be expressed as a compositionformula, InGaO₃(ZnO)_(n) (n is a natural number).

In the case where n=1 (InGaZnO₄), a crystal structure illustrated inFIG. 74A can be obtained, for example. Note that in the crystalstructure in FIG. 74A, a Ga atom and an In atom each have five ligandsas described in FIG. 71B, a structure in which Ga is replaced with Incan be obtained.

In the case where n is 2 (InGaZn₂O₅), a crystal structure illustrated inFIG. 74B can be obtained, for example. Note that in the crystalstructure in FIG. 74B, since a Ga atom and an In atom each have fiveligands as described in FIG. 71B, a structure in which Ga is replacedwith In can be obtained.

A CAAC-OS film can be formed by sputtering. The above material can beused as a target material. In the case where the CAAC-OS film is formedby a sputtering method, the proportion of an oxygen gas in an atmosphereis preferably high. In the case where sputtering is performed in a mixedgas of argon and oxygen, for example, the proportion of an oxygen gas ispreferably 30% or higher, more preferably 40% or higher because supplyof oxygen from the atmosphere promotes crystallization of the CAAC-OSfilm.

In the case where the CAAC-OS film is formed by a sputtering method, asubstrate over which the CAAC-OS film is formed is heated preferably to150° C. or higher, more preferably to 170° C. or higher. This is becausethe higher the substrate temperature becomes, the more crystallizationof the CAAC-OS film is promoted.

After heat treatment is performed on the CAAC-OS film in a nitrogenatmosphere or in vacuum, heat treatment is preferably performed in anoxygen atmosphere or a mixed gas of oxygen and another gas. This isbecause oxygen deficiency due to the former heat treatment can bereduced by supply of oxygen from the atmosphere in the latter heattreatment.

A film surface on which the CAAC-OS film is formed (deposition surface)is preferably flat. This is because roughness of the deposition surfaceleads to generation of grain boundaries in the CAAC-OS film because thec-axis approximately perpendicular to the deposition surface exists inthe CAAC-OS film. For this reason, the deposition surface is preferablysubjected to planarization treatment such as chemical mechanicalpolishing (CMP) before the CAAC-OS film is deposited. The averageroughness of the deposition surface is preferably 0.5 nm or less, morepreferably 0.3 nm or less.

Note that an oxide semiconductor film (or an oxide semiconductor layerformed using an oxide semiconductor film) formed by a sputtering methodor the like contains moisture or hydrogen (including a hydroxyl group)as impurities in some cases. In one embodiment of the present invention,in order to reduce impurities such as moisture or hydrogen in the oxidesemiconductor film (the oxide semiconductor layer) (in order to performdehydration or dehydrogenation), heat treatment is performed on theoxide semiconductor film (the oxide semiconductor layer) in areduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a raregas, or the like, an oxygen gas atmosphere, or ultra dry air (themoisture amount is 20 ppm (−55° C. by conversion into a dew point) orless, preferably 1 ppm or less, more preferably 10 ppb or less, in thecase where measurement is performed by a dew point meter in a cavityring-down laser spectroscopy (CRDS) method).

Heat treatment can eliminate moisture or hydrogen in the oxidesemiconductor film (the oxide semiconductor layer). Specifically, theheat treatment may be performed at temperature higher than or equal to250° C. and lower than or equal to 750° C., preferably higher than orequal to 400° C. and lower than the strain point of a substrate. Forexample, the heat treatment may be performed at 500° C. for 3 minutes orlonger and 6 minutes or shorter. When RTA is used for the heattreatment, dehydration or dehydrogenation can be performed in a shorttime; thus, treatment can be performed even at temperature higher thanthe strain point of a glass substrate.

After moisture or hydrogen in the oxide semiconductor film is eliminatedin such a manner, oxygen is added to the oxide semiconductor film (theoxide semiconductor layer). Thus, oxygen defects in the oxidesemiconductor film (oxide semiconductor layer), or the like is reduced,so that the oxide semiconductor layer can be an i-type or substantiallyi-type oxide semiconductor layer.

The addition of oxygen can be performed in such a manner that aninsulating film including a region where the amount of oxygen is greaterthan that in the stoichiometric composition ratio is formed in contactwith the oxide semiconductor film (or the oxide semiconductor layer),and then heated. In such a manner, excessive oxygen in the insulatingfilm can be supplied to the oxide semiconductor film (oxidesemiconductor layer). Thus, the oxide semiconductor film (oxidesemiconductor layer) can contain oxygen excessively. Oxygen containedexcessively exists, for example, between lattices of a crystal includedin the oxide semiconductor film (oxide semiconductor layer).

Note that the insulating film including a region where the amount ofoxygen is greater than that in the stoichiometric composition ratio maybe used for either the insulating film positioned on the upper side ofthe oxide semiconductor film (oxide semiconductor layer) or theinsulating film positioned on the lower side of the oxide semiconductorfilm (oxide semiconductor layer) of the insulating films in contact withthe oxide semiconductor film (oxide semiconductor layer); however, it ispreferable to use such an insulating film to both of the insulatingfilms in contact with the oxide semiconductor film (oxide semiconductorlayer). The above-described effect can be enhanced with a structure, inwhich the insulating films each including a region where the amount ofoxygen is greater than that in the stoichiometric composition ratio areused as the insulating films in contact with the oxide semiconductorfilm (oxide semiconductor layer) and positioned on the upper side andthe lower side of the oxide semiconductor film (oxide semiconductorlayer) so that the oxide semiconductor film (oxide semiconductor layer)is provided between the insulating films.

Here, the insulating film including a region where the amount of oxygenis greater than that in the stoichiometric composition ratio may be asingle-layer insulating film or a plurality of insulating films whichare stacked. Note that it is preferable that the insulating film includeimpurities such as moisture and hydrogen as little as possible. Whenhydrogen is contained in the insulating film, entry of the hydrogen tothe oxide semiconductor film (oxide semiconductor layer) or extractionof oxygen from the oxide semiconductor film (oxide semiconductor layer)by the hydrogen occurs, whereby the oxide semiconductor film has lowerresistance (has n-type conductivity); thus, a parasitic channel might beformed. Therefore, it is important that a film formation method in whichhydrogen is not used be employed in order to form the insulating filmcontaining as little hydrogen as possible. In addition, a materialhaving a high barrier property is preferably used for the insulatingfilm. For example, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, an aluminum oxide film, an aluminum nitrideoxide film, or the like can be used as the insulating film having a highbarrier property. In the case of using a plurality of insulating filmswhich are stacked, an insulating film having a lower proportion ofnitrogen such as a silicon oxide film or a silicon oxynitride film isformed to be closer to the oxide semiconductor film (oxide semiconductorlayer) than the insulating film having high barrier property. Then, theinsulating film having a high barrier property is formed to overlap withthe oxide semiconductor film (oxide semiconductor layer) with theinsulating film having a lower proportion of nitrogen providedtherebetween. With the use of the insulating film having a high barrierproperty, impurities such as moisture or hydrogen can be prevented fromentering the oxide semiconductor film (oxide semiconductor layer), theinterface between the oxide semiconductor film and another insulatingfilm, and the vicinity thereof. In addition, the insulating film havinglower proportion of nitrogen such as a silicon oxide film or a siliconoxynitride film is formed to be in contact with the oxide semiconductorfilm (oxide semiconductor layer), so that the insulating film formedusing a material having a high barrier property can be prevented frombeing in contact with the oxide semiconductor film (oxide semiconductorlayer) directly.

Alternatively, the addition of oxygen after moisture or hydrogen in theoxide semiconductor film (oxide semiconductor layer) is eliminated maybe performed by heat treatment on the oxide semiconductor film (oxidesemiconductor layer) in an oxygen atmosphere. The heat treatment isperformed at a temperature of, for example, higher than or equal to 100°C. and lower than 350° C., preferably higher than or equal to 150° C.and lower than 250° C. It is preferable that an oxygen gas used for theheat treatment under an oxygen atmosphere do not include water,hydrogen, or the like. Alternatively, the purity of the oxygen gas whichis introduced into the heat treatment apparatus is preferably greaterthan or equal to 6N (99.9999%), further preferably greater than or equalto 7N (99.99999%) (that is, the impurity concentration in the oxygen gasis less than or equal to 1 ppm, preferably less than or equal to 0.1ppm).

Further alternatively, the addition of oxygen after moisture or hydrogenin the oxide semiconductor film (oxide semiconductor layer) iseliminated may be performed by an ion implantation method or an iondoping method. For example, oxygen made to be plasma with a microwave of2.45 GHz may be added to the oxide semiconductor film (oxidesemiconductor layer).

The thus formed oxide semiconductor layer can be used as thesemiconductor layer of the transistor 601. In this manner, thetransistor 601 with extremely low off-state current can be obtained.

Alternatively, the semiconductor layer of the transistor 601 may includemicrocrystalline silicon. Note that microcrystalline silicon is asemiconductor having an intermediate structure between an amorphousstructure and a crystalline structure (including single crystal andpolycrystal). In the microcrystalline silicon, columnar or needle-likecrystal grains having a grain size of 2 nm to 200 nm, preferably 10 nmto 80 nm, further preferably 20 nm to 50 nm, still further preferably 25nm to 33 nm have grown in a direction normal to the substrate surface.Therefore, there are some cases in which a crystal grain boundary isformed at the interface between the columnar or needle-like crystalgrains.

Alternatively, the semiconductor layer of the transistor 601 may includeamorphous silicon. Alternatively, the semiconductor layer of thetransistor 601 may include polycrystalline silicon. Alternatively, thesemiconductor layer of the transistor 601 may include an organicsemiconductor, a carbon nanotube, or the like.

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 15

In this embodiment, the structures of a display panel cell having thepixel configuration described in Embodiments 7 to 14 are described withreference to FIGS. 75A and 75B.

It is to be noted that FIG. 75A is a top plan view of the display panelcell and FIG. 75B is a cross sectional diagram along a line A-A′ of FIG.75A. The display panel cell includes a signal line driver circuit 6701,a pixel portion 6702, a first scan line driver circuit 6703, and asecond scan line driver circuit 6706, which are shown by dotted lines.Further, a sealing substrate 6704 and a sealing material 6705 areprovided. A portion surrounded by the sealing material 6705 is a space6707.

Note that a wire 6708 is a wire for transmitting a signal input to thefirst scan line driver circuit 6703, the second scan line driver circuit6706, and the signal line driver circuit 6701 and receives a videosignal, a clock signal, a start signal, and the like from a flexibleprinted circuit (FPC) 6709 functioning as an external input terminal. AnIC chip (semiconductor chip including a memory circuit, a buffercircuit, and the like) 6719 is mounted over a connecting portion of theFPC 6709 and the display panel cell by chip on glass (COG) or the like.It is to be noted that only the FPC 6709 is shown here; however, aprinted wire board (PWB) may be attached to the FPC 6709. The displaydevice in this specification includes not only a main body of thedisplay panel cell but also one with an FPC or a PWB attached theretoand one on which an IC chip or the like is mounted.

Next, description is made with reference to FIG. 75B of across-sectional structure. The pixel portion 6702 and peripheral drivercircuits (the first scan line driver circuit 6703, the second scan linedriver circuit 6706, and the signal line driver circuit 6701) are formedover a substrate 6710. Here, the signal line driver circuit 6701 and thepixel portion 6702 are shown.

Note that the signal line driver circuit 6701 is formed of a p-channelor n-channel transistor such as an n-channel transistor 6720 or ann-channel transistor 6721. As for a pixel configuration, a pixel can beformed of a p-channel or n-channel transistor by applying the pixelconfiguration of FIG. 28A, FIG. 43, or FIG. 44. Accordingly, theperipheral driver circuits are formed of n-channel transistors, therebya unipolar display panel cell can be manufactured. Needless to say, aCMOS circuit may be formed of a p-channel transistor as well as ap-channel or n-channel transistor. Further, in this example, a displaypanel cell in which the peripheral driver circuits are formed over thesame substrate is shown; however, the present invention is not limitedto this. All or some of the peripheral driver circuits may be formedinto an IC chip or the like and mounted by COG or the like. In thatcase, the driver circuit is not required to be unipolar and can beformed in combination with a p-channel transistor.

Further, the pixel portion 6702 includes transistors 6711 and 6712. Itis to be noted that a source electrode of the transistor 6712 isconnected to a first electrode (pixel electrode) 6713. An insulator 6714is formed so as to cover end portions of the first electrode 6713. Here,a positive photosensitive acrylic resin film is used for the insulator6714.

In order to obtain excellent coverage, the insulator 6714 is formed tohave a curved surface having a curvature at a top end portion or abottom end portion of the insulator 6714. For example, in the case ofusing a positive photosensitive acrylic as a material for the insulator6714, it is preferable that only the top end portion of the insulator6714 has a curved surface having a curvature radius (0.2 to 3 μm).Moreover, either a negative photosensitive acrylic which becomesinsoluble in etchant by light or a positive photosensitive acrylic whichbecomes soluble in etchant by light can be used as the insulator 6714.

A layer 6716 containing an organic compound and a second electrode(opposite electrode) 6717 are formed over the first electrode 6713.Here, it is preferable to use a material having a high work function asa material used for the first electrode 6713 which functions as ananode. For example, a single layer of an indium tin oxide film, anindium zinc oxide film, a titanium nitride film, a chromium film, atungsten film, a Zn film, a Pt film, or the like, a stacked layer of atitanium nitride film and a film containing aluminum as a maincomponent, a three-layer structure of a titanium nitride film, a filmcontaining aluminum as a main component, and a titanium nitride film, orthe like can be used. It is to be noted that with a stacked layerstructure, resistance as a wire is low, good ohmic contact can beobtained, and a function as an anode can be obtained.

The layer 6716 containing an organic compound is formed by anevaporation method using an evaporation mask, or ink-jet. A complex of ametal belonging to group 4 of the periodic table of the elements is usedfor a part of the layer 6716 containing an organic compound. Besides, alow molecular material or a high molecular material may be used incombination as well. Further, as a material used for the layercontaining an organic compound, a single layer or a stacked layer of anorganic compound is often used; however, in this example, an inorganiccompound may be used in a part of a film formed of an organic compound.Moreover, a known triplet material can also be used.

Further, as a material used for the second electrode 6717 whichfunctions as a cathode and is formed over the layer 6716 containing anorganic compound, a material having a low work function (Al, Ag, Li, Ca,or an alloy thereof such as MgAg, MgIn, AlLi, CaF₂, or Ca₃N₂) may beused. In the case where light generated from the layer 6716 containingan organic compound passes through the second electrode 6717, a stackedlayer of a thin metal film with a thinner thickness and a transparentconductive layer (e.g., an indium tin oxide film, indium oxide zincoxide (In₂O₃—ZnO), or zinc oxide (ZnO)) is preferably used as the secondelectrode 6717 (cathode).

Further, by attaching the sealing substrate 6704 to the substrate 6710with the sealing material 6705, a light-emitting element 6718 isprovided in the space 6707 surrounded by the substrate 6710, the sealingsubstrate 6704, and the sealing material 6705. It is to be noted thatthe space 6707 may be filled with the sealing material 6705, as well aswith an inert gas (nitrogen, argon, or the like).

Note that an epoxy-based resin is preferably used for the sealingmaterial 6705. Further, it is preferable that these materials should nottransmit moisture or oxygen as much as possible. As a material for thesealing substrate 6704, a glass substrate, a quartz substrate, a plasticsubstrate formed of FRP (Fiberglass-Reinforced Plastics), PVF(polyvinylfluoride), polyester, acrylic, or the like can be used.

In the above manner, the display panel cell with the pixel structures inEmbodiments 7 to 14 can be obtained.

Next, a structural example of a display module which includes thedisplay panel cell described with reference to FIGS. 75A and 75B isdescribed with reference to FIG. 99.

A display module 8000 has an upper cover 8001 and a lower cover 8002with a touch panel cell 8004 connected to an FPC 8003, a display panelcell 8006 connected to an FPC 8005, a frame 8007, and a printed circuit8008 provided therebetween.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel cell 8004 and the display panel cell 8006.

The touch panel cell 8004 can be a resistive touch panel cell or acapacitive touch panel cell and may be formed to overlap with thedisplay panel cell 8006. A counter substrate (sealing substrate) of thedisplay panel cell 8006 can have a touch panel function. A photosensormay be provided in each pixel of the display panel cell 8006 and thetouch panel cell 8004 can be an optical touch panel.

The display panel cell illustrated in FIGS. 75A and 75B can be used forthe display panel cell 8006. That is, as for a pixel structure, a pixelcan be formed of a p-channel or n-channel transistor by applying thepixel structure of FIG. 28A, FIG. 43, or FIG. 44. When the peripheraldriver circuits are formed using n-channel transistors, a unipolardisplay panel cell can be formed.

The frame 8007 has a function of protecting the display panel cell 8006and functions as an electromagnetic shield for blocking electromagneticwaves generated by the operation of the printed circuit 8008. The frame8007 may function as a radiator plate.

The printed circuit 8008 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Apower source for supplying power to the power supply circuit can be anexternal commercial power source or a power source using a battery whichis additionally provided.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet

This embodiment is obtained by performing change, addition,modification, removal, application, superordinate conceptualization, orsubordinate conceptualization on part of or the whole of the otherembodiment. Thus, part of or the whole of this embodiment can be freelycombined with, applied to, or replaced with part of or the whole ofanother embodiment.

Embodiment 16

In this embodiment, examples of electronic devices are described.

FIGS. 76A to 76H and FIGS. 77A to 77D illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or an operation switch), a connection terminal 5006, asensor 5007 (sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, smell, orinfrared ray), a microphone 5008, and the like.

FIG. 76A illustrates a portable computer, which can include a switch5009, an infrared port 5010, and the like in addition to the aboveobjects. FIG. 76B illustrates a portable image reproducing deviceprovided with a memory medium (e.g., a DVD reproducing device), whichcan include a second display portion 5002, a memory medium read portion5011, and the like in addition to the above objects. FIG. 76Cillustrates a goggle-type display, which can include the second displayportion 5002, a support 5012, an earphone 5013, and the like in additionto the above objects. FIG. 76D illustrates a portable game machine,which can include the memory medium read portion 5011 and the like inaddition to the above objects. FIG. 76E illustrates a digital camerawith a television reception function, which can include an antenna 5014,a shutter button 5015, an image reception portion 5016, and the like inaddition to the above objects. FIG. 76F illustrates a portable gamemachine, which can include the second display portion 5002, the memorymedium read portion 5011, and the like in addition to the above objects.FIG. 76G illustrates a television receiver, which can include a tuner,an image processing portion, and the like in addition to the aboveobjects. FIG. 76H illustrates a portable television receiver, which caninclude a charger 5017 capable of transmitting and receiving signals andthe like in addition to the above objects. FIG. 77A illustrates adisplay, which can include a support base 5018 and the like in additionto the above objects. FIG. 77B illustrates a camera, which can includean external connection port 5019, a shutter button 5015, an imagereception portion 5016, and the like in addition to the above objects.FIG. 77C illustrates a computer, which can include a pointing device5020, the external connection port 5019, a reader/writer 5021, and thelike in addition to the above objects. FIG. 77D illustrates a mobilephone, which can include a transmitter, a receiver, a tuner of 1 segpartial reception service for mobile phones and mobile terminals, andthe like in addition to the above objects.

The electronic devices illustrated in FIGS. 76A to 76H and FIGS. 77A to77D can have a variety of functions, for example, a function ofdisplaying a lot of information (e.g., a still image, a moving image,and a text image) on a display portion; a touch panel function; afunction of displaying a calendar, date, time, and the like; a functionof controlling processing with a lot of software (programs); a wirelesscommunication function; a function of being connected to a variety ofcomputer networks with a wireless communication function; a function oftransmitting and receiving a lot of data with a wireless communicationfunction; a function of reading a program or data stored in a memorymedium and displaying the program or data on a display portion. Further,the electronic device including a plurality of display portions can havea function of displaying image information mainly on one display portionwhile displaying text information on another display portion, a functionof displaying a three-dimensional image by displaying images whereparallax is considered on a plurality of display portions, or the like.Furthermore, the electronic device including an image receiving portioncan have a function of photographing a still image, a function ofphotographing a moving image, a function of automatically or manuallycorrecting a photographed image, a function of storing a photographedimage in a memory medium (an external memory medium or a memory mediumincorporated in the camera), a function of displaying a photographedimage on the display portion, or the like. Note that functions which canbe provided for the electronic devices illustrated in FIGS. 76A to 76Hand FIGS. 77A to 77D are not limited them, and the electronic devicescan have a variety of functions.

The electronic devices in this embodiment each include a display portionfor displaying some kind of information.

Next, application examples of semiconductor devices are described.

FIG. 77E illustrates an example in which a semiconductor device isincorporated in a building structure. FIG. 77E illustrates a housing5022, a display portion 5023, a remote controller 5024 which is anoperation portion, a speaker 5025, and the like. The semiconductordevice is incorporated in the building structure as a wall-hanging typeand can be provided without requiring a large space.

FIG. 77F illustrates another example in which a semiconductor device isincorporated in a building structure. A display module 5026 isincorporated in a prefabricated bath unit 5027, so that a bather canview the display module 5026.

Note that although this embodiment describes the wall and theprefabricated bath unit as examples of the building structures, thisembodiment is not limited thereto. The semiconductor devices can beprovided in a variety of building structures.

Next, examples in which semiconductor devices are incorporated in movingobjects are described.

FIG. 77G illustrates an example in which a semiconductor device isincorporated in a car. A display module 5028 is incorporated in a carbody 5029 of the car and can display information related to theoperation of the car or information input from inside or outside of thecar on demand. Note that the display module 5028 may have a navigationfunction.

FIG. 77H illustrates an example in which a semiconductor device isincorporated in a passenger airplane. FIG. 77H illustrates a usagepattern when a display module 5031 is provided for a ceiling 5030 abovea seat of the passenger airplane. The display module 5031 isincorporated in the ceiling 5030 through a hinge portion 5032, and apassenger can view the display module 5031 by stretching of the hingeportion 5032. The display module 5031 has a function of displayinginformation by the operation of the passenger.

Note that although bodies of a car and an airplane are illustrated asexamples of moving objects in this embodiment, this embodiment is notlimited to them. The semiconductor devices can be provided for a varietyof objects such as two-wheeled vehicles, four-wheeled vehicles(including cars, buses, and the like), trains (including monorails,railroads, and the like), and vessels.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, part of the diagram or the text is takenout, and one embodiment of the invention can be constituted. Thus, inthe case where a diagram or a text related to a certain portion isdescribed, the context taken out from part of the diagram or the text isalso disclosed as one embodiment of the invention, and one embodiment ofthe invention can be constituted. Therefore, for example, in a diagramor a text in which one or more active elements (e.g., transistors ordiodes), wirings, passive elements (e.g., capacitors or resistors),conductive layers, insulating layers, semiconductor layers, organicmaterials, inorganic materials, components, devices, operating methods,manufacturing methods, or the like are described, part of the diagram orthe text is taken out, and one embodiment of the invention can beconstituted. For example, M circuit elements (e.g., transistors orcapacitors) (M is an integer, where M<N) are taken out from a circuitdiagram in which N circuit elements (e.g., transistors or capacitors) (Nis an integer) are provided, and one embodiment of the invention can beconstituted. As another example, M layers (M is an integer, where M<N)are taken out from a cross-sectional view in which N layers (N is aninteger) are provided, and one embodiment of the invention can beconstituted. As another example, M elements (M is an integer, where M<N)are taken out from a flow chart in which N elements (N is an integer)are provided, and one embodiment of the invention can be constituted.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, in the case where at least one specificexample is described, it will be readily appreciated by those skilled inthe art that a broader concept of the specific example can be derived.Thus, in the diagram or the text described in one embodiment, in thecase where at least one specific example is described, a broader conceptof the specific example is disclosed as one embodiment of the invention,and one embodiment of the invention can be constituted.

Note that in this specification and the like, a content described in atleast a diagram (or may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Thus, when a certain content is described in a diagram, thecontent is disclosed as one embodiment of the invention even when thecontent is not described with a text, and one embodiment of theinvention can be constituted. Similarly, part of a diagram that is takenout from the diagram is disclosed as one embodiment of the invention,and one embodiment of the invention can be constituted.

This application is based on Japanese Patent Application serial No.2011-145262 filed with Japan Patent Office on Jun. 30, 2011, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a transistor; a first switch; asecond switch; a third switch; a fourth switch; a capacitor; a load; afirst line; a second line; and a third line, wherein: a first terminalof the first switch is electrically connected to the first line; one ofa source and a drain of the transistor is electrically connected to asecond terminal of the first switch and a first terminal of the secondswitch; the other of the source and the drain of the transistor iselectrically connected to a first terminal of the third switch and afirst terminal of the fourth switch; a gate of the transistor iselectrically connected to a second terminal of the second switch and oneelectrode of the capacitor; the other electrode of the capacitor isdirectly connected a second terminal of the fourth switch and a firstterminal of the load; the other electrode of the capacitor is notdirectly connected to any other switch other than the fourth switch; asecond terminal of the third switch is electrically connected to thesecond line; and a second terminal of the load is electrically connectedto the third line.
 2. The semiconductor device according to claim 1,wherein the first to fourth switches are transistors.
 3. Thesemiconductor device according to claim 1, wherein the first to fourthswitches are transistors having the same polarity.
 4. The semiconductordevice according to claim 1, wherein the load is a display elementhaving a rectification property.
 5. The semiconductor device accordingto claim 1, wherein the second line is a signal line.
 6. Thesemiconductor device according to claim 1, wherein the first line is apower supply line.
 7. The semiconductor device according to claim 1,wherein the third line is a cathode line.
 8. The semiconductor deviceaccording to claim 1, wherein the transistor comprises a semiconductorlayer containing an oxide semiconductor.
 9. The semiconductor deviceaccording to claim 1, wherein the first to fourth switches aretransistors, each of the transistors comprises a semiconductor layercontaining an oxide semiconductor.
 10. A display module comprising thesemiconductor device according to claim 1 and at least one of flexibleprinted circuit and printed circuit.
 11. A display device comprising thesemiconductor device according to claim
 1. 12. An electronic devicecomprising the semiconductor device according to claim
 1. 13. Asemiconductor device comprising: a transistor; a first switch; a secondswitch; a third switch; a fourth switch; a capacitor; a load; a firstline; a second line; and a third line, wherein: a first terminal of thefirst switch is electrically connected to the first line; one of asource and a drain of the transistor is electrically connected to asecond terminal of the first switch and a first terminal of the secondswitch; the other of the source and the drain of the transistor iselectrically connected to a first terminal of the third switch and afirst terminal of the fourth switch; a gate of the transistor iselectrically connected to a second terminal of the second switch and oneelectrode of the capacitor; the other electrode of the capacitor iselectrically connected to a second terminal of the fourth switch and afirst terminal of the load; a second terminal of the third switch iselectrically connected to the second line; a second terminal of the loadis electrically connected to the third line; the second line iselectrically connected to a first circuit configured to supply a firstpotential and a second potential; the first line is electricallyconnected to a second circuit configured to supply a third potential;the third line is electrically connected to a third circuit configuredto supply a fourth potential; the first potential is lower than thefourth potential; and the second potential is used for controlling anamount of current flowing in the first wiring supplied with the thirdpotential and the third wiring supplied with the fourth potential by thetransistor.
 14. The semiconductor device according to claim 13, whereinthe first to fourth switches are transistors.
 15. The semiconductordevice according to claim 13, wherein the first to fourth switches aretransistors having the same polarity.
 16. The semiconductor deviceaccording to claim 13, wherein the load is a display element having arectification property.
 17. The semiconductor device according to claim13, wherein the second line is a signal line.
 18. The semiconductordevice according to claim 13, wherein the first line is a power supplyline.
 19. The semiconductor device according to claim 13, wherein thethird line is a cathode line.
 20. The semiconductor device according toclaim 13, wherein the transistor comprises a semiconductor layercontaining an oxide semiconductor.
 21. The semiconductor deviceaccording to claim 13, wherein the first to fourth switches aretransistors, each of the transistors comprises a semiconductor layercontaining an oxide semiconductor.
 22. A display module comprising thesemiconductor device according to claim 13 and at least one of flexibleprinted circuit and printed circuit.
 23. A display device comprising thesemiconductor device according to claim
 13. 24. An electronic devicecomprising the semiconductor device according to claim
 13. 25. A drivingmethod of a semiconductor device, the semiconductor device comprising: atransistor; a first switch; a second switch; a third switch; a fourthswitch; a capacitor; a load; a first line; a second line; and a thirdline, wherein: a first terminal of the first switch is electricallyconnected to the first line; one of a source and a drain of thetransistor is electrically connected to a second terminal of the firstswitch and a first terminal of the second switch; the other of thesource and the drain of the transistor is electrically connected to afirst terminal of the third switch and a first terminal of the fourthswitch; a gate of the transistor is electrically connected to a secondterminal of the second switch and one electrode of the capacitor; theother electrode of the capacitor is electrically connected to a secondof the fourth switch and a first terminal of the load; a second terminalof the third switch is electrically connected to the second line; and asecond terminal of the load is electrically connected to the third line,the driving method comprising the steps of: bringing the first switch,the second switch, the third switch, the fourth switch, and thetransistor into electrical conduction in an initializing step; supplyinga first potential which is an initializing potential to a secondterminal of the third switch in the initializing step; supplying a thirdpotential to the first terminal of the first switch in the initializingstep; supplying a fourth potential which is higher than the firstpotential to the second terminal of the load in the initializing step;turning off the first switch and the fourth switch in a first step afterthe initializing step; supplying a second potential which is a signalpotential to the second terminal of the third switch in the first step;turning on the first switch and the fourth switch in a second step afterthe first step; and turning off the second switch and the third switchin the second step.
 26. The semiconductor device according to claim 25,wherein the first to fourth switches are transistors.
 27. Thesemiconductor device according to claim 25, wherein the first to fourthswitches are transistors having a same polarity.
 28. The semiconductordevice according to claim 25, wherein the load is a display elementhaving a rectification property.
 29. The semiconductor device accordingto claim 25, wherein the second line is a signal line.
 30. Thesemiconductor device according to claim 25, wherein the first line is apower supply line.
 31. The semiconductor device according to claim 25,wherein the third line is a cathode line.
 32. The semiconductor deviceaccording to claim 25, wherein the transistor comprises a semiconductorlayer containing an oxide semiconductor.
 33. The semiconductor deviceaccording to claim 25, wherein the first to fourth switches aretransistors, each of the transistors comprises a semiconductor layercontaining an oxide semiconductor.